Fix the completely broken tests/testheightforwidth

Fix the completely broken tests/testheightforwidth

This was ruined, with only 1 of the 8 subwindows rendering any content.

This commit fixes the responsible errors in the embedded GtkBuilder UIs:
 • Fix broken replace by commit fb3d9022ad98049c887cec5aeffd6b73deb043ba
   of HBox with a Box having a broken orientation <property>
 • Replace VBox and [HV]Paned with GtkOrientable successors (properly!)
 • Remove use of Button:use_action_appearance, as this no longer exists

This commit also adds error reporting, in case other errors creep into
the GtkBuilder UI definitions, plus cleanup for the Builders and Windows