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.