Improve button handling in the API

[lenardg]

Currently the API provides us with button statuses only (is it pressed or is it released). The demo game scrolly-tile does some bitfield magic to keep track of what is actually done with the buttons - and not just if it is held or not.

Relevant code:

	uint16_t changed = blit::buttons ^ last_buttons;
	uint16_t pressed = changed & blit::buttons;
	uint16_t released = changed & ~blit::buttons;

It would be great if this same functionality would be part of the engine.

Instead of just blit::buttons it could be a struct of some kind, where you would have three members:

• status - to check the status of a single button, like what blit::buttons is now.
• pressed - that would tell which button was pressed since last time
• released - that would tell which button was released since last time

ALthough it might be a little bit long to write, it would increase clarity.

For example: if ( blit::buttons::pressed & blit::button::A )

[ali1234]

You can avoid the complement operation:

uint16_t released = changed & last_buttons;

[Daft-Freak]

I think I did (quite close to) this in #310?