Move Django template debugging to a separate plugin. Added Jinja2 template debugging.

This is 
a) refactoring of tracing, that moves all Django related code to plugin that is enabled on respective breakpoint add and 
b) Implementation of Jinja2 template debugging ontop of the new plugin architecture.

The resulting tracing speed should remain the same(we have measured performance of this new version) while code readability is increased by decoupling from Django tracing parts.

Patch authors: Elizaveta239 and traff

Known issues: set attribute is not implemented yet for Jinja2 and broken for Django, we will fix it afterwards.
The problem with having more then one active plugin is performance. As trace_dispatch is called very often we get performance drawback when we ask all plugins every time. So we've decided to deal with only one that is active. We can allow to have more then one active, but activating only on breakpoint add I think is required not to spoil performance.