Tweaked the automatic input mode selection

[Falcury]

• Fixed not being able to start the game from the title screen by pressing shift (it unfortunately seems impossible to distinguish whether pressing shift was meant as control input, or as part of a hotkey, so we have to assume that it was meant as control...)
• Check that the controller axis value is outside the dead zone to prevent accidental input being registered.
• Changed the switching to be immediately processed and made the switching silent. (Not sure if this is the right choice, but it feels very seamless)

[NagyD]

Fixed not being able to start the game from the title screen by pressing shift

That usage of shift is checked here: https://github.com/NagyD/SDLPoP/blob/master/src/seg000.c#L490

[NagyD]

Maybe I could merge this to master as well?