Skill check usability key

Petzi requested:

"There should be an entity to check what skills/forcepowers the player has and at one level."

New usability key: skillname,<skill>,skillvalue,<value>

skill: internal skill name (ex: "jedi_force_sense", "merc_jetpack").
skillvalue: If positive, skill must be >=. If negative, skill must be < (or <=, whatever it is for level key).

Include "jedi_side" as a hack (0 neutral, 1 light, 2 dark).