dashsink: generate init files and MPD compliant with DASH IF when using fmp4
This merge request adds the following changes to dashsink element:
- It generates and writes an initialization file, when fmp4 muxer is employed
- It adds the initialization files information to the MPD
These changes allows the dashsink to be compliant with DASH IF recommendation (the recommendation does not allow self-initializing segments). Further details can be found here: https://discourse.gstreamer.org/t/dashsink-does-not-create-a-stream-compatible-with-dash-if/3443