Clicking on an avatar shows a sliding pane with user details

[kevinaboos]

• 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.