Change when symbolic icons are forced in the application

Rather than forcing symbolic icons in GNOME, check whether only symbolic
icons are available for the current icon theme and if so, then force
the symbolic icons, otherwise use colored icons.

The test can be still inaccurate, but the gtk+ API doesn't have a way to
check whether the icon theme is symbolic-only or not.

Related to https://gitlab.gnome.org/GNOME/evolution/-/issues/1848