GtkIconTheme's headers are split between a public and a private
one. We won't expose the icon theme API at all, nor do we need
to access anything beyond what the texture cache is currently
using, so merge the private header into the implementation.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2620>
GTK4 changed icon loading significantly, it is now closely tied
to snapshots and paintables. This makes a port highly unrealistic,
so to avoid staying stuck on GTK3 forever, copy the relevant code
into the tree.
The code is unmodified except for the include names and replacing
some stray tab indentation. It is still full of GTK internals, so
it will take a while before we can actually build it.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2620>
