Started work on the `ClientModel` change handler
Added a conversion between KeySyms and strings.

This allows for easier debugging, since KeySyms can be logged.
Minor fixes to `ClientModelEvents`
More work on `ClientModelEvents`.

The relayering mechanism is partially written - the current layer change
handler just sets a flag for later.
Added a way to retrieve all icons from `XModel`.

This is necessary to do window layering, since all the icons must be
raised.
Finished implementing layer stacking.

Also, a minor formatting error was corrected in `xdata.cpp`.