* Add selection feedback for both selection area and text field

* Addressing comment

* Fixes more test

* Support keyboard selection in selectable region

* fix some comments

* addressing comments

* Can show context menus anywhere in the app, not just on text.
* Unifies all desktop/mobile context menus to go through one class (ContextMenuController).
* All context menus are now just plain widgets that can be fully customized.
* Existing default context menus can be customized and reused.

* added Magnifier for iOS and Android

* Fixes a bug when dragging selection handle sends events in wrong coordinates system

* remove comments

* addressing comments

* Use `curly_braces_in_flow_control_structures` for `widgets`

* fix comments

* fix comments

* Support global selection

* addressing comments

* add new test

* Addressing review comments

* update

* addressing comments

* addressing comments

* Addressing comments

* fix build