Fix documentation
media: don't free PurpleMedia when "init*-media" have no handler

The logic was changed so that the PurpleMedia instance isn't disposed
when "init-media" or "init-private-media" have no connected handlers.

We want the media object freed only when some signal callback function
explicitly returns FALSE, indicating an error during the initialization.
media: removed purple_media_manager_remove_private_media()

The function wasn't called from anywhere and the medias are removed only
in the dispose function of PurpleMedia where we don't care if the object
is public or private.
appdata: Don't hold a strong media reference in PurpleMediaAppDataInfo

Because of this permanent reference, PurpleMedia refcount never reached
zero and thus the objects weren't properly disposed.
media: ensure nonunique source is removed from pipeline when unlinked
media: DTMF support