core.text: Introduce `get_system_fonts_dir` and register the dirs at start.
This would allow one to simply specify a system font like font_name: 'Comic Sans MS'
core:Text: include kivy_data_dir/fonts to resources
core:text fix logic for getting system fonts for windows
core:_text_sdl2 fix for python3
Merge pull request #2673 from kivy/system_fonts

System fonts