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".
This will allow the utility scripts glib-mkenums and gdbus-codegen be
generated with the proper info in them, as build systems such as Meson
might look for shebang lines to determine the commands that need to be
called to invoke the scripts (which is necessary for calling these
scripts on standard Windows cmd.exe)
