Add weapon toggle inputs to InputManager.

New inputmanager methods:
TogglePrimary: check if Q was tapped
ToggleSecondary: check if E was tapped
NumKey: return int corresponding to pressed numkey, or -1 if none pressed
Spawn unicorns on left and right screen sides
Change unicorn sprite angling
Work on unicorn movement
Improve unicorn behavior.

Unicorns now spawn on horizontal level edges, scan vertically for the
player, and charge horizontally. They can now also be eaten by a black
hole.