Rename the app-id to org.gnome.Settings since this is what
we've been calling it for many years now. Adjust all files
that derive from the app-id, such as the desktop file, D-Bus
service file names, search providers, GSettings schemas, to
match that.
Closes https://gitlab.gnome.org/GNOME/gnome-control-center/-/issues/344
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.
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.
