Add disable-mouse-button-modifiers message

Similar to the disable-keybindings message, Sugar would like to be able
to disable this specific part of the metacity functionality without
changing the GSettings configuration (which would also affect GNOME).

Add a new metacity-message command to disable mouse button modifiers,
which ordinarily let windows be dragged around the screen when a specific
modifier is pressed.

https://bugzilla.gnome.org/show_bug.cgi?id=677115