ADFA-3597: Add the Layout Editor plugin

A visual drag-and-drop editor for Android XML layouts. Contributes a preview-slot
toolbar action (enabled only when a layout XML file is open and the last sync
succeeded), a full-screen editor with palette, component tree, typed attribute
dialogs, and a resource manager, and long-press documentation on every control.
Includes README.md and layout-editor-documentation.html.

Author: alome007

README.md

@@ -21,7 +21,7 @@ See the official [plugin documentation](https://www.appdevforall.org/codeonthego
 | [`rainbow-on-the-go/`](rainbow-on-the-go/)         | Colors matching parentheses, brackets, and braces by nesting depth, with light/dark palettes. |
 | [`compose-preview/`](compose-preview/)             | Renders Jetpack Compose `@Preview` functions on-device — no full app build or run. |
 | [`ai-literacy-course/`](ai-literacy-course/)       | Bundles Learn AI Anywhere's offline "Introduction to AI" course (26 videos + interactive activities) and plays it full-screen, fully offline. |
-| [`layout-editor/`](layout-editor/)                 | Visual drag-and-drop editor for Android XML layouts, extracted from the host into a plugin. |
+| [`layout-editor/`](layout-editor/)                 | Visual drag-and-drop editor for Android XML layouts. |
 
 ## Building a plugin