DTMF: add, and require, support for [pPxX,] and [wW] in dialstrings
DTMF: add, and require, support for [pPxX,] and [wW] in dialstrings

Handling 'w' correctly requires new API, but adding this while there are
no implementations of MultipleTones "in the wild" seems easier than
defining capability discovery.

TelepathyIM/telepathy-spec,freedesktop-unofficial-mirror/telepathy__telepathy-spec