Fixes MPLUS1p-Medium.ttf not found issue of NSZ 4.6 GUI

Older NSZ versions could corrupt the global Kivy configurations as reported in #154. This was fixed in NSZ 4.6. Some users where unable to start NSZ 4.6 GUI due to having corrupted Kivy configurations. This release automatically repairs the Kivy configuration. It does so by resetting the default_font property if corrupted. This fixes #164

This is a day 1 fix for NSZ 4.6. Please read at the NSZ 4.6 release notes instead to see what changed if you upgrade from an older version.