supporting python2 and python3 in kdevelop

[Sven Brauch]

Hmm, this is more difficult than I expected. It's not easily possible to 
separate which files need to be parsed with python 2 and which with python 3 
(they can be outside of the project, or in multiple projects, or relevant for 
both language versions like django, ...), and using both gets messy. So I 
guess switching language plugins on a per-project basis is not a good idea.

Thus the next best option is switching it per session. Do we have a place to 
provide configuration UI for a session? I guess not?
Otherwise, I could also imagine placing the UI in the project config dialog 
(which is also where people will look for it) and putting a note there that 
it'll be stored along with the session. Do you think that's an exceptionally 
bad idea?

Greetings,
Sven