multiview: moving most functions to use viewname instead of view_id

This is a one commit, and one question pull request.
@brechtvl I'm not sure whether to:

1) get rid of Render.actview entirely (and use only Render.viewname everywhere)
2) keep a mixed approach (as this pull is doing) - extended to other functions that could also work with string instead of int)
3) forget this patch and use "const char*" for the views only for the RNA/api function you suggested.

I'm inclined towards 2.