entry, spinbutton: Finish css node conversion

entry, spinbutton: Finish css node conversion

This commit creates entry and button subnodes for the buttons
in GtkSpinButton. The nodes are ordered like this for horizontal

  spinbutton
  + entry
    + image.left
    + image.right
    + progress
  + button.down
  + button.up

and like this for vertical ones:

  spinbutton
  + button.down
  + entry
  + button.up

This arrangement requires cooperation from GtkEntry to place
the entry subnodes correctly, and some small changes in the theme.

This commit also fixes progress rendering in vertical spin buttons.