Clicking on an avatar shows a sliding pane with user details (#78)

* Introduce a new UserProfileSlidingPane, which fades in from the right side
  of the screen when the user clicks on an Avatar (big or small).
  * Room avatars do not enable this action, as clicking on a room avatar
    should open the room itself instead.
  * This borrows some button designs from Moxin, which show nice SVG icons
     next to the button text.
  * These buttons are not yet functional; that will be added shortly.

* The flow of events now works like this:
  1. User clicks on an Avatar
  2. The `Avatar` widget handles that click and emits a `ShowUserProfileAction`
  3. The `RoomScreen` widget handles that action, sets the info for the `UserProfileSlidingPane`, and then shows that sliding pane.
  4. The user views the pane, and clicks the close button (or pressed back/Esc, etc), returning them to the RoomScreen.
 
* Redesigned how the Avatar works to be a bit more flexible, e.g., it now accepts
  any username or User ID string and still display the first non-'@' grapheme.
* Avatars also store data about the user they're displaying,
  which enables them to be self-sufficient when handling actions like being clicked.

* Reorganize imports and `live_design()` invocations in each module group.

* Still need to fix a few things:
  * Setting the width of the sliding pane to be a percentage of the parent window's width
  * Making sure that events are not forwarded to the hidden RoomScreen beneath the UserProfileSlidingPane
  * Hide the sliding pane if the user clicks/taps outside of its bounds (e.g., like a Modal)
  * Implement the button actions
  * Properly query and display the user's role (power level) in the current room.

README.md

@@ -115,3 +115,7 @@ These are generally sorted in order of priority. If you're interested in helping
 - [ ] User profile settings screen
 - [ ] Persistence of app state to disk
 
+
+## Known problems/issues
+ - URLs do not wrap properly; that is a Makepad-side problem.
+ - Matrix-specific links are not yet fully handled (https://matrix.to/...)