It should be possible to build both Wayland (sans D-Bus) and GNOME
versions simultaneously. Add a Wayland target, and, for consistency,
rename the GNOME target.
D-Bus support is large, and should have an option to be excluded. Move
the struct implementing D-Bus support from the dispatcher to the main
file, so it can be replaced with a stub.
The D-Bus interfaces used by Phosh are org.gnome.SessionManager for
recovering critical services from crashes and sm.puri.OSK0 for manual
on-screen keyboard activation. Add support for them.
