(pycharm can be an arse sometimes)

…ilters themselves

 - Fixes per Eldinnie's comments.
 - Fixes per warnings when running sphinx.

Context Based Handlers -> Context Based Callbacks
No longer use_context args on every single Handler
Instead set dispatcher/updater .use_context=True to use
Works with
- Handler callbacks
- Error handler callbacks
- Job callbacks
Change examples to context based callbacks so new users are not confused
Rename and move the context object from Handlers.HandlerContext to CallbackContext, since it doesn't only apply to handlers anymore.
Fix tests by adding a new fixture `cpd` which is a dispatcher with use_context=True