Ok that was a bit of a mess with the 4 build cases (combinations of
meson, autotools, vector and raster icons). I *think* this is now OK.
Basically we still need to build the colorsvg2png tool even when
installing vector icons, just for the purpose of the 2 icons
dialog-question and gimp-wilber-eek which we compile into GLib resources
from PNG images.
Also it looks like I completely forgot to add the subdir meson.build in
icons/Color/.
Freedesktop's "Thumbnail Managing Standard" defines 2 standard sizes
for thumbnails: 128x128 and 256x256 (section 5.3. Thumbnail Size).
Since the "folder" icon is used in-place of thumbnail when selecting a
folder in the GTK+ file dialog, it should also provide these default
sizes.
