test-widget: only one window

It was possible to create several windows to display different files. It
is only a test program, one window is enough. It is still possible to
open another file.
Renovate test-widget

- use a composite widget template
- replaces the menu with a sidebar, like in test-completion.
- fixes lots of warnings due to deprecated functions (GtkStock,
  GtkUIManager, etc.).
- remove the command line options; if needed those options can be added
  to the UI instead.