Video and voice calls

Support for making and receiving video and audio calls.

Also in this release:
a426d18 Fix for broken "sub" pushes
920034e Fix handling of deleted values (DEL_CHAR)
ab6e996 Expose method Topic.latestMsgVersion
7d79d03 Adding background data fetch and acknowledgements
489f7e2 Export AccessMode from Tinode
c7d0e95 Fixed some jsdoc problems
d8c16d4 Allow uploading images and files without authentication
abfbbcd Fix for invalid "this"

The full list of changes: v0.19.3...v0.20.0