session: add new Initialize method

Right now the worker interface has three methods for starting
a conversation: Setup, SetupForUser, SetupForProgram

Each of these method calls take a large number of overlapping arguments.
Extending these argument lists is painful and breaks upgrades.

This commit adds a new, fourth call, Initialize, which supercedes the
others and just takes a vardict, which is much more extensible.

https://bugzilla.gnome.org/show_bug.cgi?id=788851