Add installation functionality to Extension Manager

Functional changes to File > 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.

Main code changes from previous version:
* `ExtensionManagerModel` has been renamed to `ExtensionManager` and is now responsible for tracking the state of local extensions as well as loading the registry.
* Instead of a single view template for the whole list, there's now a template for each item, so that we can re-render individual items when the state of an extension changes.

@gruehle