Implement "text-align"

"text-align" allows setting the alignment of text, with respect to
other lines and allocated space, without requiring a reference to
the ClutterText (which is private for most widgets).
If not specified, all text is left-aligned.

https://bugzilla.gnome.org/show_bug.cgi?id=622447

src/st/st-theme-node.h

@@ -58,6 +58,13 @@ typedef enum {
     ST_TEXT_DECORATION_BLINK        = 1 << 3
 } StTextDecoration;
 
+typedef enum {
+    ST_TEXT_ALIGN_LEFT = PANGO_ALIGN_LEFT,
+    ST_TEXT_ALIGN_CENTER = PANGO_ALIGN_CENTER,
+    ST_TEXT_ALIGN_RIGHT = PANGO_ALIGN_RIGHT,
+    ST_TEXT_ALIGN_JUSTIFY
+} StTextAlign;
+
 typedef enum {
   ST_GRADIENT_NONE,
   ST_GRADIENT_VERTICAL,
@@ -149,6 +156,8 @@ int    st_theme_node_get_transition_duration (StThemeNode *node);
 
 StTextDecoration st_theme_node_get_text_decoration (StThemeNode *node);
 
+StTextAlign st_theme_node_get_text_align (StThemeNode *node);
+
 /* Font rule processing is pretty complicated, so we just hardcode it
  * under the standard font/font-family/font-size/etc names. This means
  * you can't have multiple separate styled fonts for a single item,