add support for DASH events

Submitted by A Ashley

Link to original bug (#796923)

Description

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.

Currently there is no support in qtdemux or dashdemux for these DASH events.