Split key input into two different functions:
	-one which will handle continuous key presses by checking the current state of keys of interest (perform_action_cont())
	-one which will handle single key presses by passing keydown to the function perform_action_press()
To do:
	-alter the input to ensure that only one action (keypress) will be carried out per cycle (simultaneous keypresses result in multiple actions)
	-give the character terrain interactions so that it can no longer walk through walls