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-Layout-HorizontalBoxLayout.md

@@ -0,0 +1,21 @@
+## Name
+
+GML Horizontal Box Layout
+
+## Description
+
+Defines a horizontal GUI box layout. This is a layout object that lays out widgets side-by-side horizontally.
+
+## Examples
+
+```gml
+@GUI::Widget {
+    layout: @GUI::HorizontalBoxLayout {
+        spacing: 2
+    }
+}
+```
+
+## See also
+
+-   [Layout](help://man/5/GML-Layout)