Hook up user interface for tool installation (gdiff, 194c4f9)

Add a menu item to bring up the installer dialog, set up the bindings and connections for initiating installation in response to button clicks, show a progress indicator and disable the buttons when an install is in action.

This is only the user interface; the back end has not yet been put in place.

Signed-off-by: Greg Hurrell <greg@hurrell.net>