Add support for serialising a GIcon to a GVariant and deserialising the
result back to a GIcon.
This solves a number of problems suffered by the existing to_string()
API, primarily these:
- not forcing the icon to be a utf8 string means that we can
efficiently encode a PNG (ie: just give the array of bytes)
- there is no need to ensure that proper types are loaded before using
the deserialisation interface. 'Foreign' icon types will probably
emit a serialised format the deserialises to a GBytesIcon.
We additionally clearly document what is required for being a consumer
or implementation of #GIcon.
Further patches will be required to GdkPixbuf and GVfsIcon to bring
their implementations in line with the new rules (essentially: introduce
implementations of the new serialize() API).
https://bugzilla.gnome.org/show_bug.cgi?id=688820
GBytesIcon is an icon that has a GBytes inside of it where the GBytes
contains some sort of encoded image in a widely-recognised file format.
Ideally this will be a PNG.
It implements GLoadableIcon, so GTK will already understand how to use
it, but we will add another patch there to make things more efficient.
https://bugzilla.gnome.org/show_bug.cgi?id=688820
