Bring various v4 recent improvements to next v3 version

[peaBerberian]

I worked a lot on our next-v4 branch recently which is the branch for our next v4 major versions.

Some work I have recently done on that branch can also be upstreamed to the v3 releases.
This branch thus adds:

• API: Stop sending events if the content is stopped due to a side-effect of one of the event handler.
  For example previously, if the application stopped the content during a availableAudioTracksChange event, it was possible to still receive an availableTextTracksChange event after it.
• API: send available...TracksChange events in the very unlikely scenario where tracks are added after a manifest update (for the same Period for DASH, MetaPlaylist and local contents)
• Demo: manifest and segment timeouts are now configurable in the demo page
• Demo: add description to each option, so people can understand what each does
• Demo: minor fix on the buffer size graph, I'm not sure that the issue seen in the v4 demo is reproducible in the v3 one, but it's still a fix
• Demo: Remove fake log displayer screen on the bottom of the page, as it doesn't really show anything. If you want logs, open the console
• tests: fix behavior of one of our memory tests
• a minor RepresentationStream guard, I'm unsure if this has an effect for the v3 but it is still a safety check.