Enable scroll in histogram view and curve editor (#246)

Both GthHistogramView and GthCurveEditor have histograms with event
handler code for scroll-event but the widgets are missing the
GDK_SCROLL_MASK bit for events, resulting in no scroll support. The
event handlers already exist. Adding the mask bit makes the event
handlers effective.

Author: darkowlzz

extensions/file_tools/gth-curve-editor.c

@@ -1003,6 +1003,7 @@ gth_curve_editor_init (GthCurveEditor *self)
 				| GDK_POINTER_MOTION_HINT_MASK
 				| GDK_ENTER_NOTIFY_MASK
 				| GDK_LEAVE_NOTIFY_MASK
+				| GDK_SCROLL_MASK
 				| GDK_STRUCTURE_MASK));
 	gtk_widget_show (self->priv->view);
 	gtk_container_add (GTK_CONTAINER (view_container), self->priv->view);

pix/gth-histogram-view.c

@@ -936,7 +936,7 @@ gth_histogram_view_init (GthHistogramView *self)
 	gtk_widget_show (view_container);
 
 	self->priv->view = gtk_drawing_area_new ();
-	gtk_widget_add_events (self->priv->view, GDK_BUTTON_PRESS_MASK | GDK_BUTTON_RELEASE_MASK | GDK_POINTER_MOTION_MASK | GDK_STRUCTURE_MASK);
+	gtk_widget_add_events (self->priv->view, GDK_BUTTON_PRESS_MASK | GDK_BUTTON_RELEASE_MASK | GDK_POINTER_MOTION_MASK | GDK_STRUCTURE_MASK | GDK_SCROLL_MASK);
 	gtk_widget_show (self->priv->view);
 	gtk_container_add (GTK_CONTAINER (view_container), self->priv->view);