Register quickfixes via extension point

[Bananeweizen]

• Remove old quickfixprovider extension point. It's entirely useless to register an OSGi plugin as extender of another plugin. OSGi will deal with that kind of class loading anyway.
• Instead have a new quickfix extension point, which registers a class implementing the quickfix related interface and the module that can be fixed this way.
• Completely avoids loading Checkstyle metadata for looking up quickfixes of existing problem markers.
• Fix ExplicitInitializationQuickfix throwing exceptions during the check whether it's applicable.

Fixes #315.
Fixes #561.

See the icons with the light bulb overlay here for violations with quickfixes available:

[rnveach]

@Bananeweizen Just a question, does eclipse-cs have support for 3rd party (like sevntu) quick fixes where the code is in the other library? The way you add quickfixes here make me think that is a no, but I wanted to ask.

[Bananeweizen]

Of course that works. As long as the quickfix is in a plugin, it will be found that way. Us registering our quickfixes is the same as someone else registering theirs. We only query the registered quickfixes, we don't have to deal with anything beyond that (class loading etc.). That's the reason for having the extension point mechanism in the first place, to NOT deal with classloading on your own.