Add DVB Font Downloads

[mattjuggins]

For TTML subtitle tracks it would be beneficial for content providers using dash.js to be able to specify custom fonts for use with subtitles. This is described in the DVB Dash specification ETSI TS 103 285 V1.4.1 - Section 7.2 Downloadable Fonts.

Functionality to meet the specification should be implemented, including handling DVB font download attributes in essential and supplementary property descriptors.
Fonts should also be downloaded to the browser, cleaned up, and the risk of clashing with other local/browser fonts minimised.

PR #4338 has been opened as a proposal for this functionality, which has further information on the implementation.

[dsilhavy]

Implemented in #4338