Fix focus loss on decoration destruction

InputDeviceHandler keeps track of which client has focus, and whether
the decoration or main window has focus.

Should the decoration get destroyed whilst the decoration has focus we
don't update correctly. This can happen when
"BorderlessMaximizedWindows" is set, or if you use key presses whilst
above the decoration.

cleanupDecoration (which is an odd name) is called whenever a decoration
gains or loses focus. Here we can connect for the decoration destruction
and force a focus change.

BUG: 411884