Added sound, new melee mechanics, polish

Big update for an old project! Getting it ready to be extended.

Added sound. Musical intro, ambience, movement, enemy barks (literally in the hound's case), and more.

Melee is no longer instant death. Instead you get a chance to run away, if there's cover you can move behind. So consider carefully where you end your turn! To keep it from being too easy, though, the hound gets a new "follow up" ability where once per turn, if you escape its melee attack, it will immediately chase you one tile and try again. This means that it's not enough to have an escape route; you need an escape route from your escape route! Here's a GIF of what that looks like in action:

The line-of-sight visual effects for the bunny were entirely re-done; now they're soft-edged, support diagonals, and animate (fade-in/out):

Minor gameplay changes:
Enemies "sniff" you out if you're on an adjacent diagonal behind cover and they're looking around. Unlike melee, they can't attack you, but they'll move to that position in their next turn and investigate (look around again).
You can't move on top of enemies anymore. Previously that was suicide; now the pathing algorithm just won't do it.
Enemies complete their patrol routes after they've been distracted. Previously they just went to the furthest waypoint.

There's now a start screen that identifies the various gameplay elements (cover, enemies, icons).

The cover graphics have been tweaked to hopefully be more clear.

Numerous bug fixes.