Bug 1062105 - [Keyboard] Alternate keys list overflows needs UI
 improvement.
  - Add border to multi-row alternatives char menu.
  - Adjust the positioning of the menu to align with the key row
    underneath.
Merge pull request #23851 from RudyLu/keyboard/Bug1062105

Bug 1062105 - [Keyboard] Alternate keys list overflows needs UI improvement.
r=timdream