Help: Convert to GML and propagate more errors

Converts Help's layout to GML, propagates model and icon creation
errors, and switches to the MainWidget-namespace organization
pattern seen in more up-to-date apps like TextEditor to make things
easier to maintain going forward.

Userland/Applications/Help/HelpWindow.gml

@@ -0,0 +1,50 @@
+@GUI::Widget {
+    fill_with_background_color: true
+    layout: @GUI::VerticalBoxLayout {
+        spacing: 2
+    }
+
+    @GUI::ToolbarContainer {
+        @GUI::Toolbar {
+            name: "toolbar"
+        }
+    }
+
+    @GUI::HorizontalSplitter {
+        layout: @GUI::HorizontalBoxLayout {
+            spacing: 4
+        }
+
+        @GUI::TabWidget {
+            name: "tab_widget"
+            fixed_width: 200
+            container_margins: [6]
+
+            @GUI::TreeView {
+                name: "browse_view"
+            }
+
+            @GUI::Widget {
+                name: "search_container"
+                layout: @GUI::VerticalBoxLayout {}
+
+                @GUI::TextBox {
+                    name: "search_box"
+                    placeholder: "Search"
+                }
+
+                @GUI::ListView {
+                    name: "search_view"
+                }
+            }
+        }
+
+        @Web::OutOfProcessWebView {
+            name: "web_view"
+        }
+    }
+
+    @GUI::Statusbar {
+        name: "statusbar"
+    }
+}