Find/Replace UI:
* Treat Enter as Find Next instead of closing the bar. Removes Enter key
handling and "commit" event from ModalBar.
* Move error message into popup & clean up how its shown/hidden. Make text
field border red whenever error is shown. Same for FindInFiles, as well.
* Remove redundant "Skip" button in Replace UI
* Make next/prev and Replace/All button pairs flush
* Move result-count label inside search field
* Fix all existing unit tests (removing some that are no longer applicable
with the new Enter key behavior).