Add install button and install extension when clicked
Disable install button when already installed. Needs some cleanup.
Renamed model to ExtensionManager. Made ExtensionManager the central place for finding out about extension status.
Render individual registry entries as separate views so we can update individual ones when status changes
Clarify distinction between showing the install dialog and using it to automatically do an install
Clean up handling of Install button label
Move Install Extension menu item to be a button in the Extension Manager dialog
Put extension download url in config file
Disable installation and show warning for extensions that are incompatible with the current Brackets version