Bugs and Walls [Alpha 4]

This update adds a small change to endless mode and fixes a lot of bugs.

Additions:

• Added some door variations in Endless mode as you get further in (walls either side of varying length).
• Added key count to editing palette to help with debugging.
• Cleaned up map screenshots to look more like their previews.

Fixes:

• Changed preview gfx for Slow.
• Fixed an overflow bug to do with sorting recycled sprites (recycled so many that the sorting order went into negative numbers).
• Turner Helm no longer blocks an ally swap.
• Campaign map is reloaded when dropping in / out of edit mode.
• Turned off spawn sound for spawn-on-death elements (too noisy).
• Bigger Virus resting frame (easier to tell apart from a Box when it has passives).
• Changed selection sound (sounded too farty).
• Fixed animations not being garbage collected.
• Fixed some acting elements being able to act after being destroyed.
• Fixed add-level button calling map preview and crashing the game.
• Fixed endless mode becoming unplayable after saving a level in edit mode.
• Fixed horizontal trap showing vertical trap gfx.

I hoped to add some new features and do more work on Endless mode but the amount of bugs I fixed (some which have been in the game for a long time) demanded I put out a stable build of the game before further tinkering.