dragsurface: Add compute-size signal

dragsurface: Add compute-size signal

Similarly to GdkToplevel, GdkDragSurface's compute-size should be called
by backends to query the current surface size, and should be connected
to by widget implementations (like GtkDragIcon) to report the current
size.

GdkDragSurface-backed widgets are not parented to an existing widget,
unlike popovers, and like toplevels. This means that there's nobody to
actively call gdk_drag_surface_present() to update the size, and
GdkDragSurface should do it on its own, just like GdkToplevel.