Add an utility function to change owner to the sddm user
Define qt4_add_dbus_adaptor as a wrapper to the Qt 5 counterpart

Don't duplicate code in other CMakeLists.txt files and just use the
qt4_add_dbus_adaptor macro all the time.
One CMakeLists.txt file for each subdirectory

It's easier to work with separate CMakeLists.txt files, one for each
subdirectory.