* add initial support for een.be (belgian) teletekst
* ttx-link: add ttx_link_to_string
* ttx-window.[ch]: add support for having different teletekst providers
* ttx-http: include url in error
* update Makefile.am for new provider
* ttx-provider-type-1: generic implementation of teletekst retrieval 'type 1':

  (for lack of a better name); type-1 is used at least by nos-teletekst and
  een-be flemish teletekst, probably more
* move provider implementations to providers/
* add TTXProvider, which abstracts a specific provider
* update TTXWindow for changes
* update TTXProviderMgr
* ttx.c: update for changes elsewhere
* update Makefile.am