A single icon theme can contain both regular and symbolic icons. Let's
give possibility to switch from one style to the other within GIMP
Preferences.

This won't work very well in all cases yet, especially if an icon theme
only has symbolic icons (and no regular ones) because of inconsistencies
in glib which are being fixed (patches which I submitted and which were
merged in glib on 2018-08-17).
So this will work as expected when we will bump our glib requirement to
whatever is the next release.