MSVC builds: Allow different Python versions per toolset

MSVC builds: Allow different Python versions per toolset

Use conditionals to select the Python installation, so that we can stick
closer to the default Visual Studio versions used to compile each official
Python releases.

This means by default:
-2013 builds use Python 3.4.x, which is built with 2010
-2015 and 2017 builds use Python 3.6.x, which is built with 2015.

Also rename PythonPath/PythonPathX64 in the property sheets to
PythonDir/PythonDirX64 repsectively, as PythonPath is the envvar name
where additional Python modules is searched for, so we don't want to get
confused with it.

Last but not least, distinguish between the Python interpretors that are
used on x64 and x86/32-bit builds for generating the libgtk4.manifest
file and the gdbus-generated sources, for consistency reasons.