Add Middle Click Emulation to mouse manager

This change depends on gsettings key commit in mate-desktop pull https://github.com/mate-desktop/mate-desktop/pull/91.

This adds the necessary bits to enable middle mouse click emulation to the mouse manager. I took the patch, which was committed, from gnome bug 633863 and poked at it long enough so that it works with MATE.

I tested it for a few days now and could not find any problems.

Gnome Bug 633863: https://bugzilla.gnome.org/show_bug.cgi?id=633863
Patch: https://bugzilla.gnome.org/attachment.cgi?id=173810
Gnome commit: https://git.gnome.org/browse/gnome-settings-daemon/commit/?id=a2dec1bb0f734a430db481486ce5f450a9716f3a