win32: Update MSVC projects for glib-genmarshal

glib-genmarshal is now a Python script instead of a compiled program, so
we need to:

-Remove the projects that are used to build the glib-genmarshal sources.
-Generate the full glib-genmarshal Python script from glib-genmarshal.in
-Make Python a hard build-time requirement, since we use this tool
 in many parts of the stack (and it is the case for glib-mkenums).
-Tell people in the Visual Studio build README.txt files that Python
 2.7.x or 3.x is now required for the build/"install".