Centralize computations of border and padding into StThemeNode

Rather than repeating the computation of borders in many different
widget subclasses, add helper functions:

 st_theme_node_adjust_for_height()
 st_theme_node_adjust_preferred_width()
 st_theme_node_adjust_for_width()
 st_theme_node_adjust_preferred_height()
 st_theme_node_get_content_box()

That are used in get_preferred_width()/get_preferred_height() and
allocate() methods to consistently apply the necessary adjustments.
This allows removing the StPadding type.

Queueing a relayout when the borders/padding change is moved from
st_widget_real_style_changed() to the invoking code to allow access
to the old StThemeNode for comparison. (Should this be added as
a parameter to the signal?)

Borders are included in the geometry adjustments, but borders
are not yet drawn.

https://bugzilla.gnome.org/show_bug.cgi?id=595993