vtray

Update README.md with information about the needed dependencies and tested platforms.

ci: add `#pkgconfig --cflags gdk-pixbuf-2.0` to fix running on ubuntu-20.04

fix running (runtime symbol conflict when gtk2 and gtk3 are used at the same time; link only gtk2, do not link gtk3)

ci: add `submodules: true`

ci: retry on github