WIP: dashdemux: add support for DASH events that signal end of stream
Section 5.10.3 (Inband Event Signalling) of the MPEG DASH specification defines a new box type "emsg" that is used to convey events. These events can be used to signal changes in the DASH stream, convey metadata or provide application specific information.
Section 5.10.4 (DASH-specific events) defines a method for an in-band signal to indicate that a live DASH stream has finished.
This MR adds support for listening to these messages and setting the end time of the stream if signalled in an event box. When the end time is reached, end_of_manifest is set, a GROUP_DONE event and an EOS event is sent to every active pad. The reason for a GROUP_DONE event is to allow downstream elements (such as qtdemux) to detect the difference between the end of a segment and the end of the stream.
Fixes #492 (closed) Fixes #762 (moved)