/usr/include/libgladeui-2.0/gladeui/ is in libgladeui-dev 3.20.0-2+deb9u1.
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/usr/include/libgladeui-2.0/gladeui/glade-app.h
/usr/include/libgladeui-2.0/gladeui/glade-base-editor.h
/usr/include/libgladeui-2.0/gladeui/glade-builtins.h
/usr/include/libgladeui-2.0/gladeui/glade-catalog.h
/usr/include/libgladeui-2.0/gladeui/glade-cell-renderer-icon.h
/usr/include/libgladeui-2.0/gladeui/glade-clipboard.h
/usr/include/libgladeui-2.0/gladeui/glade-command.h
/usr/include/libgladeui-2.0/gladeui/glade-cursor.h
/usr/include/libgladeui-2.0/gladeui/glade-debug.h
/usr/include/libgladeui-2.0/gladeui/glade-design-view.h
/usr/include/libgladeui-2.0/gladeui/glade-displayable-values.h
/usr/include/libgladeui-2.0/gladeui/glade-editable.h
/usr/include/libgladeui-2.0/gladeui/glade-editor-property.h
/usr/include/libgladeui-2.0/gladeui/glade-editor-skeleton.h
/usr/include/libgladeui-2.0/gladeui/glade-editor-table.h
/usr/include/libgladeui-2.0/gladeui/glade-editor.h
/usr/include/libgladeui-2.0/gladeui/glade-inspector.h
/usr/include/libgladeui-2.0/gladeui/glade-name-context.h
/usr/include/libgladeui-2.0/gladeui/glade-palette.h
/usr/include/libgladeui-2.0/gladeui/glade-placeholder.h
/usr/include/libgladeui-2.0/gladeui/glade-project.h
/usr/include/libgladeui-2.0/gladeui/glade-property-class.h
/usr/include/libgladeui-2.0/gladeui/glade-property-label.h
/usr/include/libgladeui-2.0/gladeui/glade-property-shell.h
/usr/include/libgladeui-2.0/gladeui/glade-property.h
/usr/include/libgladeui-2.0/gladeui/glade-signal-class.h
/usr/include/libgladeui-2.0/gladeui/glade-signal-editor.h
/usr/include/libgladeui-2.0/gladeui/glade-signal-model.h
/usr/include/libgladeui-2.0/gladeui/glade-signal.h
/usr/include/libgladeui-2.0/gladeui/glade-utils.h
/usr/include/libgladeui-2.0/gladeui/glade-widget-action.h
/usr/include/libgladeui-2.0/gladeui/glade-widget-adaptor.h
/usr/include/libgladeui-2.0/gladeui/glade-widget.h
/usr/include/libgladeui-2.0/gladeui/glade-xml-utils.h
/usr/include/libgladeui-2.0/gladeui/glade.h