Several improvements to plugins.

+ Now the help and parameter checking methods are optional.
+ Now plugins specify directly which classes they want, instead of using
our internal constants. This method is still mandatory, mostly because I
don't want people using a default of None to receive a bunch of stuff they
won't act upon. I want plugin developers to pay attention to what data types they
expect.
+ Removed a bunch of outdated and wrong comments copypasted from old versions of the
plugin template.
+ Removed some strange code.
+ Some other small fixes I don't remember right now.