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
Make corners rounded.
Replace selected style with an overlay check, make the close button
smaller to match. Move slideshow indicator to the lower left corner.
Set margins in css.
Update the unused symbolic to match the svg with a baked-in shadow,
actually use it. Bundle it in gresources instead of installing, fix the
style class name. Fix margins - the close button was overriding them.
This is a dedicate class to generate the preview. Not only
it renders the pixbuf of the wallpaper, but it also adds
lock screen or desktop frames on top of it. It also monitors
the time to keep the lock screen preview label always updated.
