Bug 951920: fix visuals for alternative character popups

 - minor modifications to keyboard.css for menu sizing, including
   negative offsets for the menu position.
 - change the way that alternative menu overflows are handled
   since the negative offsets mean they  can now overflow on both sides
 - change the way that events are retargeted when the menu is displayed
   to properly handle the negative offsets
 - change the way that we ensure that the first alternative is always
   displayed when the menu first pops up
 - remove a bunch of unused code for the old layout menu
Merge pull request #15016 from davidflanagan/bug951920

Bug 951920: fix visuals for alternative character popups r=rudylu