Text track switching not working fine for Fragmented Text

Created by: epiclabsDASH

When working with fragmented text tracks, switching across different tracks is not always working. More specifically, whenever the switch implies aborting a previous request that is still in progress, new requests are never done.

Reason behind this is when a request is aborted from TextController, https://github.com/Dash-Industry-Forum/dash.js/blob/2451c6bf4161fe39e31a8645b7a430dc3512aaa1/src/streaming/text/TextController.js#L148, the event FRAGMENT_LOADING_ABANDONED is not triggered and then, StreamController doesn't set isFragmentProcessingInProgress to false what causes no new requests for subsequent fragments are done.

Removing calling abort from TextController fixes reported issue although is not a real solution (after switching text track, for a few second captions of previous selected text track could be shown). We need to find out the way to let scheduleController knows a request for a fragmentedText chunk has been aborted.

I have verified this issue exists at least from dash.js 2.3 to nightly builds.