Systray icon improvements

* Hide rather than minimize the MainWin on click (this is standard behavior)
* Bring window to front if it's obscured by other windows, rather than minimize it
  This works with KDE integration and on Windows only. Sorry pure Qt users, but I don't
  feel like reimplementing the X11 voodoo required for finding out our state :/
* Properly restore the old position after unhiding the window
* Select highlighted buffer if a highlight is pending
* Fix some corner cases where minimize/restore failed before

Kudos to the KSystemTrayIcon developers for showing me how to stab some manners into window managers :)