month-view: allow changing months with scrolling

It is very natural and intuitive to try and change the current date
by scrolling. The current implementation of the Month view, however,
blatantly ignores scroll events.

Fix that by hearing to the scrolling events and changing the month
when the scroll is enough.

src/gcal-utils.c

@@ -63,6 +63,8 @@ month_item[12] =
   MON_12
 };
 
+#define SCROLL_HARDNESS 10.0
+
 G_DEFINE_BOXED_TYPE (icaltimetype, icaltime, gcal_dup_icaltime, g_free)
 
 gint
@@ -844,3 +846,37 @@ get_alarm_trigger_minutes (GcalEvent          *event,
 
   return diff;
 }
+
+gboolean
+should_change_date_for_scroll (gdouble        *scroll_value,
+                               GdkEventScroll *scroll_event)
+{
+  gdouble delta_y;
+
+  switch (scroll_event->direction)
+    {
+    case GDK_SCROLL_DOWN:
+      *scroll_value = SCROLL_HARDNESS;
+      break;
+
+    case GDK_SCROLL_UP:
+      *scroll_value = -SCROLL_HARDNESS;
+      break;
+
+    case GDK_SCROLL_SMOOTH:
+      gdk_event_get_scroll_deltas ((GdkEvent*) scroll_event, NULL, &delta_y);
+      *scroll_value += delta_y;
+      break;
+
+    /* Ignore horizontal scrolling for now */
+    case GDK_SCROLL_LEFT:
+    case GDK_SCROLL_RIGHT:
+    default:
+      break;
+    }
+
+  if (*scroll_value <= -SCROLL_HARDNESS || *scroll_value >= SCROLL_HARDNESS)
+    return TRUE;
+
+  return FALSE;
+}