Multisrc caption sync - #895

Related issue: #892

Changes in this PR:

• Fix caption synchronization for playback in multi-source canvases for all browsers
• Fix duration update in Safari and other browsers in iOS context: when the linked IA Manifest is loaded into Ramp in Safari the duration always shows up as 1 minute (duration of each source in the Canvas) even though the total duration of that Canvas is 13 minutes. This happens due the 'loaded' event cycle triggering a native 'durationchange' event which overwrites the manually set player duration.
• Fix playback looping of second source in a multi-source Canvas in Chrome and Safari: In Chrome and Safari once the playback reaches the end of the second source in the Canvas in the linked IA Manifest it keeps playing that source in a loop. This is happening due a timing issue of the 'ended' event in these specific browsers where it fires before the source switch.