The previous code was crashing when used as the returned classes array
would have been invalid after the first deletion. So if a 2nd class
would be deleted, invalid memory might have been referenced.
This function is getting called a lot. Statistics for the gtk3-demo
listbox example show most calls with 0-4 classes. Unrolling the cases
a bit brings the instruction count in callgrind from 93M to 52M.
GtkCssNodeDeclaration is a new struct with copy-on-write semantics.
It encapsulated the properties used to define a node in the CSS tree.
The idea is to use it in various places for caching, in particular as
key in hash tables.
