Fixed some commenting irregularities.

I added periods to the end of every section of every Doxygen comment.
Made WMConfig::get_config_path() const.
Wrote a header describing what the window manager shares.
Fixed a couple issues with the Makefile.

Specifically, not all the targets required that the `obj` directory exist.
Added some constants to the common header to avoid too much magic.
Removed the current_desktop parameter from the shared data pool.

This will actually be managed by a different entity.
Wrote the Client and ClientManager code.

This code also is the basis for a lot of additional utility classes:
 - LayerManager
 - DesktopManager
 - MoveResizeManager
 - FocusManager