paned: Don't override properties when buildable
We set GtkPaned:{shrink,resize}-{start,end}-child when adding children as buildable.
This overrides whatever value the UI definition had set for the properties. Even if not set explicitly, the properties have a default value anyway.
This fixes an issue I found while porting nautilus to GTK 4, as the start child was shrinkable even though :shrink-start-child was set to FALSE in the .ui file.