RicImportGridAndSummaryEnsembleDialog: Add dialog and extract shared tree helpers

- Add new RicImportGridAndSummaryEnsembleDialog for searching and selecting
  grid and summary ensemble files
- Extract firstLevelItems, setCheckedStateChildItems, findItemsMatching and
  populateComboBoxHistoryFromRegistry to public static methods on
  RicRecursiveFileSearchDialog to eliminate duplication

Author: magnesj