Add directories containing only prerendered symbolic icons to index.theme

If some directory contains no rendered icons or svgs, then the code we use to detect the list of directories to put in the index.theme file will not add them.

These dirs will still have files in them (generated by the gtk-encode-symbolic-svg), but since they are not in the index.theme they will not be used.

We fix this by generating a .placeholder file in such directories so that the index.theme generation works.