The widget path machinery assumes that we always have types,
and without this change, it will start spewing warnings when
we start to introduce node names.
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.
