1. Remove set_icons property
2. Make it a GtkWidget subclass
3. Add gtk_fishbowl_set_creation_func()
4. Make the widgetbowl use the new benchmarking infrastructure of the
fishbowl
We now properly use GdkFrameTimings and can therefor check for dropping
even a single frame in the history (of 16 frames or 1/4 of a second).
Once that happens, we immediately stop adding new items to the bowl.
A side effect is that the number of icons is now a *lot* smaller.
And have a priv->display instead of a priv->screen.
Includes turning gtk_menu_set_screen() into gtk_menu_set_display(),
because that function just forwards to its window.
