This change updates `SelectableRegion`s right-click gesture to match native platform behavior.

Before: Right-click gesture selects word at position and opens context menu (All Platforms)
After: 
- Linux, toggles context menu on/off, and collapses selection when click was not on an active selection (uncollapsed).
- Windows, Android, Fuchsia, shows context menu at right-clicked position (unless the click is at an active selection).
- macOS, toggles the context menu if right click was at the same position as the previous / or selects word at position and opens context menu.
- iOS, selects word at position and opens context menu.

This change also prevents the `copy` menu button from being shown when there is a collapsed selection (nothing to copy).

Fixes #117561

Rename Sample classes

Remove 1745 decorative breaks

* Make Flex,Row,Column const for real

* dart fix --apply

* fix snippets

* fix integration test

* add comment

* Support keyboard selection in selectable region

* fix some comments

* addressing comments