textbuffer: add unit tests for the clipboard

It tests gtk_text_buffer_paste_clipboard(),
gtk_text_buffer_copy_clipboard() and gtk_text_buffer_cut_clipboard() in
various situations, including when GtkTextTags are applied to the
selection.

The last test didn't pass.
Improve doc of gtk_text_buffer_paste_clipboard()

It didn't explain the behavior when there is a non-empty buffer
selection.