Switch to GDBus

[NovaAndrom3da]

According to the official dbus-glib documentation, it is obsolete and replaced by GDBus.

[lukefromdc]

Here's the current status of a stalled partial migration to gdbus:

#170
migrated session saving

#171
migrated
mate-session/test-client-dbus.c

In
https://github.com/mate-desktop/mate-session-manager/pull/208/files
we see that in
test-inhibit.c
the code was migrated to gdbus but
#include <dbus/dbus-glib.h>
was left in place at the top of the file. Building without it seemed to
work fine, implying we are not using it anymore in this file

Apparently some of the other gdbus PR's did this too or for whatever reason
these files are not using glib-dbus functions

In test-inhibit.c in main.c gsm-inhibitor.c main.c gsm-client.c gsm-util.c gsm-presence.c
the only remaining reference to dbus-glib is an include, removing
it did not stop the build from finishing

in gsm-systemd.c gsm-manager.c gsm-consolekid.c and dbus-client.c (probably the big one...)
we are actually still using dbus-glib functions, removing the include breaks the build

[lukefromdc]

Uploaded https://github.com/mate-desktop/mate-session-manager/tree/gdbus-finish including all of the removed includes that allow the build to finish. This is the first half of what is needed for this, contributions for the other four files that still use glib-dbus welcome