Extra features, changes and fixes

Fixed GetPlayerBehaviours(client, BehaviourType:filter) so it doesn't
add copies to the internal array (woops)

Added native Gamma_ForceStopGameMode, which can be used to stop the game
mode forcefully, it calls the internal StopGameMode(forceful=true) -
which still needs to restart the round

Added cvar gamma_target_filters, which sets the amount of targetfilters
created by gamma when a game mode starts, 0=no filters, 1=behaviour type
target filters, 2=beahviour type and behaviour target filters

Added forward Gamma_PluginDetected, which is called when gamma detects a
plugin for the first time, use this to register the game mode

Changed Gamma_OnGameModeCreate to also be called when a plugin has just
been detected, so only create behaviours in there from now on

Changed the templates to reflect the modifications made to gamma