PixelPaint: Use the parallel ImageProcessor to apply filters

The main advantage of this change is that heavy-weight filters do not
lock up the GUI anymore.

This first cut has several flaws:
- We do not account for modification of the referenced images while the
  filter is running. Depending on the exact filter behavior this might
  have all sorts of weird effects. A simple fix would be to show a
  progress dialog to the user, preventing them from performing other
  modifications in the meantime.
- We do not use the image processor for previews. Preview behavior has a
  couple of other considerations that are intentionally not addressed in
  this commit or pull request.

Userland/Applications/PixelPaint/ImageProcessor.cpp

@@ -20,7 +20,7 @@ void FilterApplicationCommand::execute()
     m_filter->m_editor->gui_event_loop().deferred_invoke([strong_this = NonnullRefPtr(*this)]() {
         // HACK: we can't tell strong_this to not be const
         (*const_cast<NonnullRefPtr<Layer>*>(&strong_this->m_target_layer))->did_modify_bitmap(strong_this->m_target_layer->rect());
-        strong_this->m_filter->m_editor->did_complete_action();
+        strong_this->m_filter->m_editor->did_complete_action(String::formatted("Filter {}", strong_this->m_filter->filter_name()));
     });
 }