fixes issue #2018 - control video from speaker view in main window

This PR fixes issue #2018

It allows the speaker view to control/sync the video in the main window (only one way).

The following operations are supported:

• play/puase
• seeking
• playback rate (speed 1x, 1.5x, ...)

There is currently the issue that all instances of reveal.js (across all tabs) are synced as a BroadcastChannel is used. This could be solved by adding a unique session id to the reveal instance.

How it works:

After the plugin is loaded all video elements are found and listeners are attached to the events (play, pause, ...). In the listener the xpath for the video element is queried and a command is send to the other instance (main view) which received the command and the xpath. Via the xpath the same element is found and the command is executed.