started work on a simple registrar service

Using the kded module 'appmenu' is a bit cumbersome. It also crashes when the
window has no menu entries. So i started a simple implementation of the
registrar dbus service.
implement registrar dbus service

I added the real functionality to the dbus service and also found the real
reason why the kded module 'appmenu' crashes: If the window isn't registered,
GetMenuForWindow returns an empty service name and an empty object path. An
assert in libdbus checks if the object path is valid and an empty path fails.
This aborts the kded module.

qmenu_registrar returns a dbus error when the window isn't registered. This
error should be handled gracefully in qmenu_hud. At the moment nothing is
shown.
show a nice message when window isn't registered

qmenu_hud shows now dmenu with only one entry "no menu entries exported" when
the current window has no menu entries or hasn't registered with the registrar
dbus service.
Merge branch 'appmenu_registrar'
added install targets

Install qmenu_hud and qmenu_registrar to CMAKE_INSTALL_PREFIX/bin.