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 (not finished)

- 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.).