Windows: Add icons and icons_geom to make.bat

This adds support for building the icons from make.bat
unlike bash there is no passing environment variables
on the command line.

The scripts go out of their way to locate both blender
and inkscape however if they are not found, the user is
given a helpful error message telling them how to set
the variables.

Although some extra help can be given there, if your
normal build is a 2019 full build running

make 2019 full icons

will help it find the blender executable as well.

finally if you know the name of your build folder
running

make builddir build_windows_Lite_x64_vc16_Release icons

will also work, if all fails you can point directly to
the blender executable by running

set BLENDER_BIN=c:\where\blender\lives\blender.exe

before running make icons or make icons_geom

The python scripts needed some small modifications since
without the PATHEXT, SystemRoot and SystemDrive
environment variables python will not initialize properly
on windows. (Not blender related, even mainline python
won't start without those)