Differentiate complementary by primary icons usage

[mfranzke]

We should probably differentiate complementary icon usage, like e.g. an icon next to a text in which the text provides meaning even without the icon and it's usage is only meant to give an additional signal, vs. icons that provide a meaning themselves without any other text next to them like e.g. in a checkbox for indeterminate or checked state.
Specifically in the latter case we might want to display the icon with a glyph instead of a ligature as we do it for all the others to provide some content even if the icon font couldn't get loaded.