Allow using the desktop font as UI interface font.

[rago666]

I am coming from Musicpod #62. It would be extremely nice if flutter snaps can pick up the default font of the desktop and use it as their interface font. I have pick up on some things on Musicpod app. It uses Baloo Da 2 and Kohinoor Bangla fonts to display my Bengali Music. Both of this font family was downloaded by me and not shipped with the distro.Below font is Kohinoor Bangla.

@Feichtmeier suggested bundling popular fonts but in my opinion this won't be a good solution. From my time browsing Unixporn the most popular font in people's setup is proprietary SF-Fonts (San Francisco, SF UI, SF Pro) from apple. Which I don't think yaru can legally bundle. Above I have shown an example how apps can use fonts that are not bundled. It would be really amazing if this functionality is expended to UI font as well.

[Feichtmeier]

At least we could bundle Cantarell as it is the default GNOME font, IMO