Add installation functionality to Extension Manager:
* "Install" buttons now appear in each registry item.
* Already-installed extensions have a disabled install button (that shows "Installed").
* Extensions with incompatible Brackets version requirements have a disabled install button and a warning message.
* File > Install Extension... has been moved out of the main menu, and is now an "Install from URL..." button on the Extension Manager dialog.
Code review fixes:
* Made tests more robust by listening for status changes instead of just waiting for file access to complete
* Scrubbed real names out of mock data
* Comment fixes
Merge pull request #3470 from adobe/nj/extension-list-install

Add installation functionality to Extension Manager