The update of the find/replace overlay placement and size while a
repaint for its target editor is being processed (e.g., in order to
handle minimizing/maximizing the target editor) is currently executed
just-in-time. Other updates, reacting to movements and resizes of the
shell, are processed asynchronously. The just-in-time execution leads to
deadlocks on GTK.

With this change, all update operations for the size and position of the
overlay are performed via asynchronous executions scheduled via the
Display. In addition, changing visibility for the overlay is only
performed if the currently visibility state does not fit.

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/2138