Reimplement lesser-used mouse controls

Reimplemented several mouse controls by way of the following settings in
doomretro.cfg:
* mouse_strafe: hold mouse button while moving mouse to strafe
left/right, set to "none" by default
* mouse_forward: press mouse button to move forward, set to "none" by
default
* dclick_use: double click the above forward or strafe mouse button to
perform use action