After the :can-focus change in the previous commit, widgets
need to set suitable focus and grab_focus implementations
to implement the desired focus behavior.
This commit does that for all widgets.
Replace uses of gtk_css_style_get_value with direct access,
throughout the tree. We don't replace all uses, just those
where we are dealing with a fixed property. Be careful to
handle the currentColor special case for color properties.
This commit introduces GtkPicture, which is supposed to complement
GtkImage.
GtkImage will be adapted to always display an icon, while
GtkPicture displays regular imagery.