Unified the definitions for Make-Shortcut in @tasks.
Moved the Python 2.7.13 GUID to $Python GUID, since it is used in regkey and
Check-Pythoninstall.
For the uninstall dismissed the batch variable %UsrFolder%, since it was
used only once and can be derived from %installFolder%. This had also to be
corrected in the uninstall script generation in install_python_gtk.ps1.
In Resetprefs.cmd I changed the text to reflect that also the downloaded Gtk
themes would be deleted.
In uninstall.cmd I changed the rd command for the themes folder to use the
correct rd syntax.