Since [1], GIO supports generic serialization and deserialization of a
GIcon into a GVariant. This is also implemented by GdkPixbuf and could be
used instead of our homegrown code for it.
This commit adds support to another 'icon' key in the metas dictionary
returned by applications for it. The previous 'gicon' and 'icon-data'
keys are still parsed and supported as before, but are now deprecated.
[1]
https://git.gnome.org/browse/glib/commit/?id=c16f914b40c749b938490a4e10a3c54ec1855c42https://bugzilla.gnome.org/show_bug.cgi?id=698761
17df668186
