Add support for MATE DE

[fohrloop]

Notes:

org.gnome.SessionManager

• Looks like Mate might be providing the org.gnome.SessionManager dbus service: mate-session/org.gnome.SessionManager.xml
• The news for mate-session-manager 1.12.0 say: "Switch to org.gnome.SessionManager"
• The mate-session-manager 1.12.0 was released Nov 3, 2015
• Here is the diff for 1.11.0...v1.12.0.

org.mate.SessionManager

• There was a service called "org.mate.SessionManager", but that was replaced with "org.gnome.SessionManager". See commit 9582afb40caf49a2386f2bed740d9e708494312f and b01c41715ddad682c2e0d2a5eac8fa0deb91c5e8
• Similar mentions about switching to org.gnome.SessionManager in mate-session-manager 1.11.0 notes, but the news do not have entry for 1.11.0. Perhaps that was never released..?

org.mate.ScreenSaver

what is this?