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

src/st/st-types.h

@@ -37,29 +37,6 @@
 
 G_BEGIN_DECLS
 
-#define ST_TYPE_PADDING               (st_padding_get_type ())
-
-typedef struct _StPadding             StPadding;
-
-/**
- * StPadding:
- * @top: padding from the top
- * @right: padding from the right
- * @bottom: padding from the bottom
- * @left: padding from the left
- *
- * The padding from the internal border of the parent container.
- */
-struct _StPadding
-{
-  gfloat top;
-  gfloat right;
-  gfloat bottom;
-  gfloat left;
-};
-
-GType st_padding_get_type (void) G_GNUC_CONST;
-
 typedef enum {
   ST_ALIGN_START,
   ST_ALIGN_MIDDLE,