If CSS values are queried from a widget, recompute them if necessary. Do
not emit style-updated until the validation phase however.
This way, we don't run into performance traps when style-update causes
invalidations that cause new style-updated to be emitted.
Instead of directly requesting the layout phase, register a tick
callback. This is what the docs suggest for animations and it's what we
need for the next commit.
- GtkCssWidgetNode
for style contexts owned by a widget
- GtkCssPathNode
for style contexts using a GtkWidgetPath
- GtkCssTransientNode
for nodes created with gtk_style_context_save()/restore()
