While GtkEventController implementations today are all GtkGesture, it is
possible to create a GtkEventController manually. This is an extrac check
to ensure we only add gestures to the list.
https://bugzilla.gnome.org/show_bug.cgi?id=774760
The sidebar in the object page was using up more space than it deserves.
Replace it with a combo box in the subheader. Now that we have more space,
put the CSS node tree and the style properties in the same page again.
The way this code is written, we know that there is an item in
the hash table and the iter_next call will give it to us, so
no need to check the return value. Annotate the call to tell
coverity.
