community/font-manager: upgrade to 0.9.0

https://github.com/FontManager/font-manager/releases/tag/0.9.0
https://github.com/FontManager/font-manager/releases/tag/0.8.9

0.8.9 resolves compile errors with Vala that led to rebuild failure on a 3.21 builder (error log).

The application identifier was changed in 0.9.0, so asset filenames in font_viewer() were updated accordingly.