WIP: Improve css state tracking

The main goal in this branch is to reduce the amount of style recomputation that is done on frequent state changes.

Numbers from widget-factory with Adwaita:

2247 nodes

Change	Before	After
class	612	612
name	2246	2246
id	2	2
first-child	148	148
last-child	156	156
state	2247	2245
hover	2247	562
disabled		859
backdrop		1080
selected		579
sibling-name	63	63
sibling-state	51
sibling-disabled		51
parent-class	586	586
parent-name	788	788
parent-id	2	2
parent-first-child	78	78
parent-last-child	78	78
parent-state	2226	115
parent-hover	5	5
parent-disabled		91
parent-backdrop		4
parent-selected		144

The important number here is parent-state, since any node with parent-state in its changes will recompute its style when the state of the toplevel (or any other ancestor) changes, e.g. when going to backdrop, or when moving the mouse out.

The number goes from 2226 to 115.