This adds the Visual Studio projects and related support files so that
one is able to use the projects to build gtksourceview using MSVC, as
well as generate the introspection files using MSVC.

Also add a math.h compatibility header for builds on Visual Studio <= 2012
so that there is an implementation for round() for these compilers, which
is taken from GDK-Pixbuf.

https://bugzilla.gnome.org/show_bug.cgi?id=759838