Css style split

This implements an idea that has been discussed on and off for a while:

Split the values in GtkCssStaticStyle in related groups, and put them into refcounted structs that can be shared with parent styles or initial value singletons. In particular, we can have a fast path for totally empty lookups.

Here are some numbers:

8535 lookups 	65.20% empty
core          	79.71% shared
background    	79.87% shared
border        	77.63% shared
icon          	89.16% shared
outline       	90.30% shared
font          	89.90% shared
font variant  	99.65% shared
animation     	99.96% shared
transition    	92.51% shared
size          	76.94% shared
other         	97.59% shared