Add splash_screen_duration to GUIApplication class.

[jonathanrocher]

Adding a new attribute splash_screen_duration to control the duration for which the splash screen is shown upon application start.

[corranwebster]

If I'm guessing what you want, is for the splash screen to persist for at least a minimum amount of time so it is visible to the user.

Probably the right way of doing this is to instead modify start_event_loop to do something like

    def start_event_loop(self):
        if self._splash_screen is not None:
            self.invoke_after(
                self.splash_screen_duration * 1000,  # milliseconds
                self.self._splash_screen.close,
            )
    ...

which avoids locking up the app, and also allows the user to dismiss by clicking on it.

[corranwebster]

This halts all processing for a number of seconds, and in particular the event loop will not be running so the app will be locked-up, which is generally not a good thing. At a minimum, this should occasionally be processing events.

[corranwebster]

This should be a float.