Remove CodeMirror references from SyncFS-editor

Not to make it depend on submodules.
(Alternatively we can import all CodeMirror tree in the repo, but for
simplicity I chose not.)
Remove unused SyncFS-editor assets (e.g. old icon images)
Bump the SyncFS-editor version in manifest.json
Merge pull request #231 from kinu/remove_codemirror_ref

Remove CodeMirror references from SyncFS-editor (recut)