[SDL Widget, Button] Add 'WS_COUNT' and implement a preserved 'background' on buttons so that the text can be changed.
[SDL Widget] Change widget state prefix from WIDGET_ to WS_.
[makefile] Change execution instruction to also jump directories so that the res folder can be picked up.
[Game Manager] Change 'New Game' button text to "Continue" after starting a game.

Also, remove the unused anonymous enum ("MM_").