Since the Meson build files are now updated to support building on
Visual Studio, Meson is now the supported method of building
GtkSourceView using Visual Studio, so it's time to drop the Visual
Studio projects.

The Meson build files are more comprehensive as:
-It automatically builds the introspection files and 'installs' them, if
 GObject-Introspection is found.
-The test programs are built and can be run for the build process.
-It is much easier to build directly from a GIT checkout.