main/postmarketos-ui-xfce4: set background for login greeter
Actually Xfce4 is capable to use the "draw-user-backgrounds" option of "slick-greeter", applying the users wallpaper on the greeter.
However, as it turns out, the users background in pmOS Xfce4 is set at first login of the user. Therefore, the greeters background is black at the first boot.
Therefore set the greeters background to the default postmarketOS wallpaper.
This would need to be backported to v24.12 branch.