gdk: Add macros for left, middle and right buttons

https://bugzilla.gnome.org/show_bug.cgi?id=668688

docs/reference/gdk/gdk3-sections.txt

@@ -759,6 +759,9 @@ GDK_PRIORITY_EVENTS
 GDK_PRIORITY_REDRAW
 GDK_EVENT_PROPAGATE
 GDK_EVENT_STOP
+GDK_BUTTON_PRIMARY
+GDK_BUTTON_MIDDLE
+GDK_BUTTON_SECONDARY
 
 <SUBSECTION>
 gdk_events_pending

gdk/gdkevents.h

@@ -94,6 +94,37 @@ G_BEGIN_DECLS
  */
 #define GDK_EVENT_STOP          (TRUE)
 
+/**
+ * GDK_BUTTON_PRIMARY:
+ *
+ * The primary button. This is typically the left mouse button, or the
+ * right button in a left-handed setup.
+ *
+ * Since: 3.4
+ */
+#define GDK_BUTTON_PRIMARY      (1)
+
+/**
+ * GDK_BUTTON_MIDDLE:
+ *
+ * The middle button.
+ *
+ * Since: 3.4
+ */
+#define GDK_BUTTON_MIDDLE       (2)
+
+/**
+ * GDK_BUTTON_SECONDARY:
+ *
+ * The secondary button. This is typically the right mouse button, or the
+ * left button in a left-handed setup.
+ *
+ * Since: 3.4
+ */
+#define GDK_BUTTON_SECONDARY    (3)
+
+
+
 typedef struct _GdkEventAny	    GdkEventAny;
 typedef struct _GdkEventExpose	    GdkEventExpose;
 typedef struct _GdkEventVisibility  GdkEventVisibility;