Add a recent section, composed by an additional GtkFlowBox and
a separator, and code to hide it when there are no recent
wallpapers.
To the popover menu, add a new "Remove Background" button that
is only visible when clicking on recent backgrounds.
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.
It will become essentially what CcBackgroundChooserDialog,
but without a dialog and organized differently. Right now,
it is as minimal as it could be and only shows wallpapers.
As we grow it, it may cover pictures and even colors.
