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 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
Add "showCursorWhenSelecting" pref
fix Issue #8953- add Extension Manager support for localized extension package metadata.
update localization example for reference
update localization readme for reference
as requested per code review
changes per code review
Merge remote-tracking branch 'origin/master' into bchin/localize-package-metadata
per code review, overlay first by shortlang, then by lang.
Merge remote-tracking branch 'origin' into bchin/localize-package-metadata
whitelisting properties to overlay; remove keyword concatenation with root language
add top-level key to package.json to avoid collisions
Merge remote-tracking branch 'origin/master' into pflynn/dragdrop-errors

* origin/master: (138 commits)
  Update strings.js
  rtc:panels to panes
  Updated by ALF automation.
  less and scss unit tests
  Update strings.js
  Implement Pane Headers
  Update strings.js
  Nit: change case in function name.
  remove unused var
  Update strings.js
  Update strings.js
  Make Split View Panes Resizable
  Update strings.js
  always show the status bar
  remove selection arrow
  fix fullpath check when deleting file
  Fix: toggling Extension Manager long/short description would always show the string for the installed version of an extension (if installed), even when looking at the Available tab. Use info from the current view-model rather than making absolute references to ExtensionManager's model.
  Fix CSSUtils to work better with vendor-prefixed properties
  Added unit tests
  Updated by ALF automation.
  ...

Conflicts:
	src/document/DocumentCommandHandlers.js
	test/spec/DocumentCommandHandlers-test.js
Fix bug #9096 (No error messages when dragging invalid files onto Brackets) -
The split view changes made pre-open filtering of dropped files stricter.
But actually, filtering of drag & dropped files isn't needed at all now
that images can go in working set -- so just remove it.

Fix bug where legacy DocumentManager "currentDocumentChange" was fired
needlessly for null->null cases. Fix stale docs for this event.

Re-enable most of the image-preview unit tests in DocumentCommandHandlers-test,
since that feature is implemented again. ("currentDocumentChange" fix fixes
the one test that wasn't already passing).
changes per code review
adding unit tests
Code review cleanups:
- Rename 'files' to 'paths' in cases where it's really just a string
- Use toBeFalsy() in tests instead of explicitly requiring null (although
they were passing so far, it does make them more fragile)
- Remove unused dependencies in DragAndDrop
Fix bug #3963 (Home/End goes to start/end of entire line, not just current
line-wrapped segment) -- Use new CodeMirror command that joins "goLineLeft"
(line-wrap aware Home) with the "smart home" indent-aware behavior we still
want.