GTK+ 3 theming for emoji completion uses border-spacing, which GTK+ 3 lacks

Commit 65bb238a picked the CSS from master, so now we get errors at runtime about the invalid property name in gtk-3-24 (I can trigger them by opening the Inspector on any old program), albeit only after regenerating, which that commit had not done.

My vote would go to backporting border-spacing as it's an extremely useful property, but I don't know if anyone will want to do that. If not, some alternative way of spacing the children will be needed. In my own GTK+ 3 apps, I've done some impressive and/or horrifying things to emulate border-spacing (albeit verbosely and with constraints on what is possible), but there is probably a nicer, simpler way in this particular case.

Btw, this is only in Adwaita; HC appears to have been skipped from this theming update, not that that is anything really new!