Implement "clear" method for most of collections

This patch just adds .clear() method to most of RNA collections such as modifiers list, vertex groups and so.

Method is NOT added to:
- Collections which doesn't have new/remove methods, also not added to collections which have got new/remove commented in code.
- The whole animation-specific module. It's a bit complicated data structure here, prefer to spend more time on this and make more tests.