Visual C++ Projects: Add rules to generate GTK DBus sources

Visual C++ Projects: Add rules to generate GTK DBus sources

Since commit 7c2a5072 the gtkdbusgenerated.[c|h] are not included in the
dist tarball and thus have to be generated, which broke the Visual C++
builds.

This patch adds property sheets and custom build rules for the Visual C++
projects so that gtkdbusgenerated.[c|h] will be generated upon building the
GTK+ DLL sources.

This also tells people building GTK+ from the projects that they need to
have Python 2/3 installed and the Python interpretor needs to be in their
PATH before building GTK+ from the projects.