Targets and some code tweaks

Today I've put in place targets which when hit with an arrow will operate a tile (like a gate or lift).

I've also tweaked a bit the level class so that I can distinguish easier between tiles that are collidable, targetable, destructable, static, etc

I will be working on different character classes next ;)
Can't wait to get these done, so I can start working on the lobby system which will be a bit of a headache but fun and educational (unless I encounter some other flash bug or undocumented feature)