How to symbolic icon fallback to fullcolor

[Weng Xuetian <wengxt gmail com>]

Hi,
I'm trying to implement an extension, need to display some icon within it.

The icon isn't provided by extension but some other application.
Related application send an icon name via dbus to the extension,
extension will display the icon with the default icon theme. The
related application might also provide symbolic icon, maybe not, or
maybe not the full set of all needed icon, so is there anyway to use
fullcolor as a fallback option? That means if there is corresponding
symbolic icon available then use symbolic one, if not, use the
fullcolor one.

Thanks!