UI: Fix issue #20: Search in the editor really slow

* Instead of selecting all the rows which match the search string,
  the tree view now highlights those rows which match and are visible.
* If the user presses [enter], the first matching row (if one exists) will
  be scrolled into view and selected.
* Continually pressing enter will cycle through each match, in the same way
  that clicking the "Next" button works.
* If there are no matching items when the user presses [enter], the background to the search textbox will turn red.
  It will restore to its previous color when the user next edits the text.