Add "Show in OS" context menu item to open a folder in Finder/Explorer.
Create new ProjectManager.getSidebarSelectedItem() API, and rename
getSelectedItem() -> getTreeSelectedItem() for clarity.
Merge remote-tracking branch 'origin/master' into pflynn/open-in-os

Conflicts:
	src/command/Menus.js
	src/document/DocumentCommandHandlers.js
Pass full path of files to showOSFolder() - not just parent folder's path -
so file can be shown selected
Code review comments: fix bad merge, only log warning for actual errors
Code review: switch back to a single public ProjectManager.getSelectedItem()
API, which now covers the working set selection too. Seems like a fairly
safe API change - this may even be how people assumed it worked before.

Also: add logging for a ProjectManager error case that was silently ignored
before.
Merge pull request #2128 from adobe/pflynn/open-in-os

Add "Show in OS" context menu item to open a folder in Finder/Explorer.