Obsoletes GtkHandleBox starting with Gtk 3.4

No replacement is provided for GtkHandleBox. Toolbars are just temporarily disabled. Fixed toolbars are to be restored before releasing.
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Have fixed toolbars starting with Gtk 3.4

This is because GtkHandleBox is deprecated starting with Gtk 3.4 and no replacement is provided. See #90520d850c86ce00010629e6f2cf625f6b8ed166.