Notify the user when the image is too large for the filter

Use the message box to tell the user what to do in order to apply the filter
to an image larger than 512x512, and the info banner to notify him if he
still taps on the image.

gallerytiltshiftplugin/gallerytiltshiftplugin.cpp

@@ -129,6 +129,8 @@ bool GalleryTiltShiftPlugin::receiveMouseEvent(QGraphicsSceneMouseEvent* event)
             d->m_focusPosition.setY(event->pos().toPoint().y());
             performEditOperation();
             return true;
+        } else {
+            showInfoBanner("Plugin disabled for this image size");
         }
     }
     return false;
@@ -176,6 +178,14 @@ void GalleryTiltShiftPlugin::activate()
         if (d->m_validImage) {
             showInfoBanner("Tap on an area to keep it focused");
         } else {
+            showMessageBox("Tilt Shift plugin limitations",
+                           "Gallery Tilt Shift plugin is currently limited to small images (512x512)<br />"
+                           "For a given image:"
+                           "<ol>"
+                           "<li>Scale it or crop it</li>"
+                           "<li>Save it with a different name</li>"
+                           "<li>Apply the filter to the new one</li>"
+                           "</ol>");
             GalleryTiltShiftWidget* widget = static_cast<GalleryTiltShiftWidget*>(toolBarWidget());
             widget->setEnabled(d->m_validImage);
         }