workspaceThumbnail: move event handling to ThumbnailsBox

Currently, click and drop events are handled by each WorkspaceThumbnail
instance. With the introduction of the workspace cut and the request
to extend the reactive area of the workspace selector to the edge
of the monitor, it becomes more convenient to do all the event handling
inside ThumbnailsBox, even if this requires some manual layout computation.

https://bugzilla.gnome.org/show_bug.cgi?id=643319