MSVC Builds: Rework Introspection Build

The current approach of building the introspection files for ATK works, but
is often cumbersome as one needs to set many environmental variables before
launching a solution file, which runs a Windows batch script to generate
the .gir/.typelib files.  It was also possible to hand-run the batch script
from the Visual Studio command prompt, but even more environmental
variables need to be set.

This changes the approach to build the introspection files using an NMake
Makefile (but elimating from the Visual Studio Project Files the part to
build the introspection files) to:
-Make it clearer to the person building the introspection files what
 environmental variables are needed, specifically for PKG_CONFIG_PATH and
 MINGWDIR and CFG (formerly CONF).  Setting stuff like VSVER, PLAT and
 BASEDIR is no longer required, which was a bit clunky.
-Allows some more easier flexibility on the build of the intropsection
 files.

This also cleans up the gen-file-list.py file as it can share parts in the
recently-added msvcfiles.py