Use a kmixcore library (shared) to contain the files in core/ and dbus/
used by kmix, kmixd and kmixctrl.  Also use a kmixgui library (static)
to wrap up the many files in gui/ used by kmix.

Collect installation together in CMakeLists.txt for each target and its
associated files (desktop, etc).

Move miscellaneous files to desktop/ subdirectory, eliminate some now
unused.