First approximation of LanguageLayer in prefs.
Remove dead code.
Some tests and re-work of the implementation.

Layer made state-less, just as PathLayer.
Document/Clean up.
Merge remote-tracking branch 'upstream/master' into kai/fix-6558-language-layer
Address review comments.
Break out query parsing from setting the query in the model so FindInFilesUI doesn't need to touch the model before the search starts (for #6966)
Merge remote-tracking branch 'upstream/master' into kai/fix-6558-language-layer
Merge remote-tracking branch 'upstream/master' into kai/fix-6558-language-layer

Conflicts:
	src/document/DocumentManager.js
Show cause of error when opening drag-&-dropped files fails.
Plumb the error code back from the FILE_OPEN command, and display errors
in the existing bullet list error dialog used by DragAndDrop.
add link to troubleshooting page to live preview error messages
restore string
fix typo
Add unit test to verify that FILE_OPEN command now returns error code when
it fails.

Also, cleanup: group image-related tests into a single describe() instead
of a separate one per it().
Merge remote-tracking branch 'origin/master' into pflynn/dragdrop-errors

And clean up blank-line spacing in DocumentCommandHandlers-test
change for code review
Navigate through Extension Manager tabs via Ctrl-(Shift)-Tab
Using box-shadow to stop links for shifting a few pixels when tabbed.
Remove unused vars
Add references to utils/Global back