Base: Extend and improve the GML documentation

This is a bundle of changes to the "new" GML documentation that I just
noticed two hours ago.
- Fix a bunch of typos, wording and style
- Rework layout object documentation (they're not widgets!)
- Document most of the common properties
- Finish (as for now) GML syntax documentation
- Extend GML usage explanation
- Add symlink "GML" so that man gml works
- Add a categorized GML page list to the introduction man page
- Cross-link much more

Much of the editing of existing docs is clearing up incorrect or
imprecise statements.

Note that because of massive changes in some places, git won't recognize
renames as such.

Base/usr/share/man/man5/GML-Introduction.md

@@ -4,9 +4,9 @@ GUI Markup Language (GML)
 
 ## Description
 
-GML is Serenity's graphic user interface markup language. GML files are human-readable and have no easily detectable filemagic. The format is strongly influenced by QML, the Qt Modeling Language.
+GML is Serenity's graphic user interface (GUI) markup language. GML files are human-readable text files and have no easily detectable filemagic. The format is strongly influenced by QML, the Qt Modeling Language.
 
-It allows you to easily define GUI interfaces for your applications. It is easy to learn and link to your C++ code.
+It allows you to easily define GUI interfaces for your applications. It is easy to learn and use in C++.
 
 You can easily add GML files to your project in Hack Studio either using
 
@@ -15,3 +15,64 @@ You can easily add GML files to your project in Hack Studio either using
 Or right clicking on a folder in the TreeView and using
 
 `New > GML File`
+
+## See also
+
+-   [gml-format(1)](help://man/1/gml-format) formats your GML files.
+-   [GML Playground(1)](help://man/1/Playground) is an interactive GML creation tool.
+
+### List of GML manpages
+
+-   [Using GML](help://man/5/GML-Usage)
+-   [GML syntax](help://man/5/GML-Syntax)
+-   Extending GML
+    -   [Define properties](help://man/5/GML-Define-property)
+    -   [Define widgets](help://man/5/GML-Define-widget)
+-   GML object and property reference
+    -   [Core::Object](help://man/5/GML-CoreObject)
+    -   Layouts
+        -   [HorizontalBoxLayout](help://man/5/GML-Layout-HorizontalBoxLayout)
+        -   [VerticalBoxLayout](help://man/5/GML-Layout-VerticalBoxLayout)
+    -   Widgets
+        -   [Breadcrumbbar](help://man/5/GML-Widget-Breadcrumbbar)
+        -   [Button](help://man/5/GML-Widget-Button)
+        -   [Calendar](help://man/5/GML-Widget-Calendar)
+        -   [CheckBox](help://man/5/GML-Widget-CheckBox)
+        -   [ColorInput](help://man/5/GML-Widget-ColorInput)
+        -   [ComboBox](help://man/5/GML-Widget-ComboBox)
+        -   [Frame](help://man/5/GML-Widget-Frame)
+        -   [GroupBox](help://man/5/GML-Widget-GroupBox)
+        -   [HorizontalProgressbar](help://man/5/GML-Widget-HorizontalProgressbar)
+        -   [HorizontalSeparator](help://man/5/GML-Widget-HorizontalSeparator)
+        -   [HorizontalSlider](help://man/5/GML-Widget-HorizontalSlider)
+        -   [HorizontalSplitter](help://man/5/GML-Widget-HorizontalSplitter)
+        -   [IconView](help://man/5/GML-Widget-IconView)
+        -   [ImageWidget](help://man/5/GML-Widget-ImageWidget)
+        -   [Label](help://man/5/GML-Widget-Label)
+        -   [ListView](help://man/5/GML-Widget-ListView)
+        -   [MultiView](help://man/5/GML-Widget-MultiView)
+        -   [OpacitySlider](help://man/5/GML-Widget-OpacitySlider)
+        -   [PasswordBox](help://man/5/GML-Widget-PasswordBox)
+        -   [Progressbar](help://man/5/GML-Widget-Progressbar)
+        -   [RadioButton](help://man/5/GML-Widget-RadioButton)
+        -   [ScrollableContainerWidget](help://man/5/GML-Widget-ScrollableContainerWidget)
+        -   [Scrollbar](help://man/5/GML-Widget-Scrollbar)
+        -   [Slider](help://man/5/GML-Widget-Slider)
+        -   [SpinBox](help://man/5/GML-Widget-SpinBox)
+        -   [StackWidget](help://man/5/GML-Widget-StackWidget)
+        -   [Statusbar](help://man/5/GML-Widget-Statusbar)
+        -   [TableView](help://man/5/GML-Widget-TableView)
+        -   [TabWidget](help://man/5/GML-Widget-TabWidget)
+        -   [TextBox](help://man/5/GML-Widget-TextBox)
+        -   [TextEditor](help://man/5/GML-Widget-TextEditor)
+        -   [Toolbar](help://man/5/GML-Widget-Toolbar)
+        -   [ToolbarContainer](help://man/5/GML-Widget-ToolbarContainer)
+        -   [Tray](help://man/5/GML-Widget-Tray)
+        -   [TreeView](help://man/5/GML-Widget-TreeView)
+        -   [UrlBox](help://man/5/GML-Widget-UrlBox)
+        -   [ValueSlider](help://man/5/GML-Widget-ValueSlider)
+        -   [VerticalProgressbar](help://man/5/GML-Widget-VerticalProgressbar)
+        -   [VerticalSeparator](help://man/5/GML-Widget-VerticalSeparator)
+        -   [VerticalSlider](help://man/5/GML-Widget-VerticalSlider)
+        -   [VerticalSplitter](help://man/5/GML-Widget-VerticalSplitter)
+        -   [Widget](help://man/5/GML-Widget)