Implement cross overview drag & drop

The gnome-panel allows the user to hover over a tasklist entry
while draging to activate a minimized or obscured window and drop onto it.

Implement a similar behaviour by allowing draging to the activities button or
the hotcorner (and thus opening the overview), which allows the user to
activate any window (even on different workspaces) as a drop target.

https://bugzilla.gnome.org/show_bug.cgi?id=601731

src/shell-global.h

@@ -132,6 +132,8 @@ void shell_global_run_at_leisure (ShellGlobal         *global,
 void shell_global_play_theme_sound (ShellGlobal       *global,
                                     const char        *name);
 
+void shell_global_init_xdnd (ShellGlobal *global);
+
 G_END_DECLS
 
 #endif /* __SHELL_GLOBAL_H__ */