This is a major rework on how images are loaded and stored.
Unfortunately, this is so entangled that doing each change
as an atomic step is pretty much unfeasible.
The first major change is that BgSource now returns a GListStore
instead of a GtkListStore. This is necessary for us to bind it
to GtkFlowBox, and pretty much signals we're not using any of
the tree/icon views anymore.
Second, the thumbnail factory was moved to BgSource itself. We
only create factories for large thumbnails, so it's not needed
to handle each one of them individually.
At last, switch CcBackgroundChooser to GtkFlowBox, and adjust
the signals we connect to.
We were assuming that setting stock-size would affect the emblems in
GEmblemedIcons, but not the icons themselves. This is a bit weird.
GtkCellRendererPixbuf:gicon is meant to work with
GtkCellRendererPixbuf:stock-size, and this was only working so far
because GTK_ICON_LOOKUP_FORCE_SIZE was not being used when loading
the icon.
Let's composite the emblems ourselves so that we don't have to depend
on this quirky interpretation of stock-size.
Unfortunately, we can not directly use the pixbufs because they are
unaware of the scale factor and GTK+ will scale them on HiDpi
displays. Since our pixbufs already have enough pixels to work well
with such devices, scaling them further will lead to giant, fuzzy
thumbnails. Hence, we use GtkCellRendererPixbuf:surface with the
scale factor codified in it.
https://bugzilla.gnome.org/show_bug.cgi?id=732375
It now is a full GObject, will be able to track the original
XML file for a wallpaper, as well as which values are set in it.
This will allow us to restore wallpapers properly, and only offer
editing options when those aren't already hard-coded inside the
item.
