deprecate old plugin functions, add replcements

[Geokureli]

fixes #2917

There's really no reason to force FlxG.plugins to have only one instance of any type, but we can't just change the functionality of this whole system, so for now I'm:

• deprecating add and removeType
• add addPlugin, which doesn't check existing types. addIfUniqueType which does. Also removeAllByType
• changed some formatting, docs and args