window-list: Disconnect drag and drop handlers

https://bugzilla.gnome.org/show_bug.cgi?id=737486
window-list: _pointerInTray was renamed to _pointerInNotification in bug 695800

https://bugzilla.gnome.org/show_bug.cgi?id=737486
window-list: Refactoring to use an Extension object

Move the global state into a new Extension object. This is in
preparation for adding more logic to the Extension object.

https://bugzilla.gnome.org/show_bug.cgi?id=737486
window-list: Move messageTray patching to the WindowList class

Move messageTray patching form the Extension object to the WindowList
class. Moreover, only do the patching if the window list is on the bottom
monitor. This refactoring will make it easier to have several instances
of WindowList (one on each monitor).

https://bugzilla.gnome.org/show_bug.cgi?id=737486