UI: Improve processing time of vogleditor_apicalltreeitem by pre-calculating the local row index
* This value will have to get updated if new rows (API calls) can be added / removed, which may undo this optimization,
but when investigating a slowness during searching, the code was often found in this function.
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.
Merge branch 'master' of https://github.com/PeterLValve/vogl