<Qcompound_text_with_extensions>: New variable.

<Qcompound_text_with_extensions>: New variable.
(syms_of_xselect): Intern and staticpro it.
(selection_data_to_lisp_data): Run post-read-conversion on decoded
selection text.
(lisp_data_to_selection_data): If next-selection-coding-system is
compound-text-with-extensions, set the type of selection to be
compound-text.