.. due to gdk_pixbuf_scale() with themes using the pixbuf engine

Make GimpDisplayShell a subclass of GtkEventBox, so that it gets its
own window, isolating its events from those of its ancestors.

In particular, the "expose" event handler of GtkNotebook, which the
shell is a child of in SWM, is particularly slow with themes that
use the pixbuf engine.  If the notebook and the shell use the same
window, this can cause notable, and somtimes severe, lag when the
rulers or scrollbars are updated frequently, such as when rapidly
moving the cursor.