The checkmark indicating the active background based on GtkFlowBox
selection does not work when changing the selection with the arrow keys.
Moreover, it is not set when the panel opens.
We fix this by sending the active CcBackgroundItem to the chooser, so
that it can add an "active-item" CSS class to the true active item.
Fixes#1756#2505
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.
