Moved fake manager creation and initial configuration into the proxy generator (Part I)

This is the first part of PR #372 (we split it up into 2 PRs as mentioned there).

It solves #371 partially, namely that the default behavior is available inside the c'tor call. Therefore it also solves the bug #368 (see the `when_faking_a_class_which_calls_virtual_property_members_in_constructor` test case).
A variation on a theme: in addition to the interface and method name seeming bland or vague to me, so too do the comments. Nothing here is wrong as such, but after reading this file, I don't feel like I know anything more about the interface than I did from seeing the file name.