When multiple editors have an open find/replace overlay and a resize
event happens (e.g., by minimizing/maximizing the window), the overlays
of all editors are unhidden by accident. This was a result of
introducing hide functionality to the overlay for the case that the
target editor is too small to show the overlay, which conflicts with the
hiding functionality for the case that an editor is invisible.

This change streamlines the visibility-related functionality of the
find/replace overlay to be handled at a single place. In addition, all
repositioning and resizing events are only processed when an overlay is
actually visible. For invisible overlays, the repositioning and resizing
is now performed when they become visible again. In addition, a
unnecessary layouting and positionining operations when creating the
overlay are removed as the same operations are performed again when
opening the overlay anyway.

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/2059