Convert GtkComboBox to GtkDropDown.
Changed the invalid XML entry to use error underline instead of the custom CSS background. Unfortunately, the upstream error CSS does not apply to a TextView, so this was the better alternative.
Fixed alignment on the <iq> popover.
Add a bit of vertical padding and suggested action CSS in popovers.
Inline some objects in the .ui now that <property> can contain children.

Consequently, the popover is closed with gtk_menu_button_popdown instead of gtk_popover_popdown. I don't know if it makes a difference, but it seems better to use the parent's method instead of the child's.

Opened XMPP console, picked some of the automated entries, changed to invalid XML to check the invalid styling.