Remove unneeded popover re-styling

It used to be that the rounded corners in the popovers wouldn't work
over a GtkClutter window. This seems to work better now, albeit not
perfect so remove the special casing.

https://bugzilla.gnome.org/show_bug.cgi?id=731498
LayersPopover: layout adjustments

See to it that the margins are the same in the layerpopover as in the
searchpopover. Also make the radio buttons centered, and go back to
having the default border-radius (since the popovers are rounded too).

https://bugzilla.gnome.org/show_bug.cgi?id=731498