Fix several SemDom search UI features

* Put initial cursor in the search box
* Refactor to pull out FwCancelSearchButton
* Implement FwCancelSearchButton in Collect Words
* Fix bug where search box and tree were at odds
* Fix usage count stuff in SemDom Chooser and Collect Words

Change-Id: Idb6aa973abcf69ddf5b085906a86a65de62be759