Implemented Find-Replace Overlay.

In response to issue eclipse-platform#1090. Goals for this overlay are a simple
and modern UI/UX for finding/replacing in eclipse.

-- [Modern Find/Replace] Refactor FindReplaceDialog

The FindReplaceDialog's business-logic is now handled in the
FindReplacer.
The FindReplacer communicates using a FindReplaceStatus object which is
polled by FindReplaceDialog as needed and allows for displaying correct
messages in the Interface.

This Refactoring is required so that the business-logic can be reused in
the new FindReplace overlay, as proposed in issue eclipse-platform#1090

Author: Maximilian Wittmer