add missing GDK_EVENT_*

closes #336

src/lib.rs

@@ -156,3 +156,9 @@ pub const BUTTON_MIDDLE: u32 = gdk_sys::GDK_BUTTON_MIDDLE as u32;
 
 /// The secondary button. This is typically the right mouse button, or the left button in a left-handed setup.
 pub const BUTTON_SECONDARY: u32 = gdk_sys::GDK_BUTTON_SECONDARY as u32;
+
+// Used as the return value for stopping the propagation of an event handler.
+pub const EVENT_STOP: u32 = gdk_sys::GDK_EVENT_STOP as u32;
+
+// Used as the return value for continuing the propagation of an event handler.
+pub const EVENT_PROPAGATE: u32 = gdk_sys::GDK_EVENT_PROPAGATE as u32;