[Rich text editor] Add plain text mode and new attachment UI (#7459)

* Add new attachments selection dialog

* Add rounded corners to bottom sheet dialog.

Note these are currently only visible in the collapsed state.
- [Google issue](https://issuetracker.google.com/issues/144859239)
- [Rejected PR](https://github.com/material-components/material-components-android/pull/437)
- [Github issue](https://github.com/material-components/material-components-android/issues/1278)

* Add changelog entry

* Remove redundant call to superclass click listener

* Refactor to use view visibility helper

* Change redundant sealed class to interface

* Remove unused string

* Revert "Add rounded corners to bottom sheet dialog."

This reverts commit 17c43c9188.

* Remove redundant view group

* Remove redundant `this`

* Update rich text editor to latest

* Update rich text editor version

* Allow toggling rich text in the new editor

* Persist the text formatting setting

* Add changelog entry

dependencies.gradle

@@ -101,7 +101,7 @@ ext.libs = [
         ],
         element     : [
                 'opusencoder'             : "io.element.android:opusencoder:1.1.0",
-                'wysiwyg'                 : "io.element.android:wysiwyg:0.2.1"
+                'wysiwyg'                 : "io.element.android:wysiwyg:0.4.0"
         ],
         squareup    : [
                 'moshi'                  : "com.squareup.moshi:moshi:$moshi",