[dbus] Move well-known D-Bus names/paths to lipstickdbus.h

This makes it easier for external applications to use Lipstick via D-Bus
by depending on the devel package and using defined names from the
header file instead of hardcoding/copying the values everywhere.

Also makes sure that in case a D-Bus service name goes away or changes,
the depending code will either use the new name (by depending on the
header file) or fail to build from source, instead of just silently
failing at runtime.
[launchermodel] Cleanup D-Bus handling, temp launcher removal timer