Trap Invisibility, Fixes#892
* Changed `traps_setting` config values.
* 0 = Always visible (default)
* 1 = Enable invisibility in versus maps (GVG/PVP/BG)
* 2 = Enable invisibility in all maps
* Added skill unit flag `UF_HIDDEN_TRAP = 0x20000` for trap invisibility. By default, there are (renewal) Hunter traps with this flag:
* HT_SKIDTRAP
* HT_LANDMINE
* HT_ANKLESNARE
* HT_SHOCKWAVE
* HT_SANDMAN
* HT_FLASHER
* HT_FREEZINGTRAP
* HT_TALKIEBOX
* Added some skills and effects that can reveal hidden traps. By default, Detect, Sight, Ruwach, and Improve Concentration can reveal hidden traps.