Add a TrackMouse(false) on ScreenInteractive to disable mouse events (#726)

When mouse events are enabled, it is not possible to select text in the
terminal and copy it somewhere else. This could be usefull for some
applications if they don't need to handle mouse events.

Add a function on the ScreenInteractive class to disable grabbing of
mouse events so that it is e.g. possible to select text in the user
interface. The function needs to be called on the screen object before
starting the application loop if such a behaviour is desired.