Improve entity description

Fixes issue: #109 (closed)

Also updates descriptions for: xonotic-data.pk3dir!542 (merged)

• GOLD_KEY, SILVER_KEY and OPEN_ONCE will still be supported by code, but will be considered Legacy won't be shown in the GUI. Use itemkeys and wait instead.
• target_speaker: GLOBAL is deprecated, set atten to -1 instead
• remove item_key1 and item_key2, use item_key instead
• func_pointparticles, func_conveyor, trigger_conveyor, misc_laser, trigger_gravity, trigger_music can (and should) be controlled by relays
• add func_sparks, target_kill, target_levelwarp, target_heal
• misc_laser now uses beam_color instead of colormod