"Live File Preview" menu item should be checked when connected

[njx]

The "Live File Preview" menu item acts like a toggle, so it should be checked while we're connected. (Comments in the code make me think this was originally intended to work, but the checkmark stuff wasn't in the menus yet, and we forgot to go back and update it once it was implemented.)

[pthiess]

Reviewed - Starter bug.

[ckarande]

I am beginning with Brackets, and thought of starting by helping out on this starter level bug (I already signed the CLA). Can you please verify the fix for this issue in pull request from ckarande:issue1561.

[njx]

Fixed by @ckarande. Closing.