bell handling

[jerch]

(Coming from #3314.)

Consolidate bell handling as follows:

• Strip sound output handling from xterm.js.
• Propagate onBell as interface to hook into the bell (make sound output an outer env problem). Here integrators can do whatever they want or suits their purpose.
• Move current direct sound output to a default bell addon, that keeps working in browsers as now, but listens on onBell. If demanded, we can later extend the addon by other bell metaphors, like a visual bell and such.

This cannot be done until next major release version (v5), as it involves changes to the terminal options.

[Tyriar]

It might not even be worth having a bell addon but just giving an example sound in the docs? The sound addon one is a different thing imo

[jerch]

Yes a bell-addon on its own is not very useful imho. My thoughts are - remove the sound service in v5 from core repo and just go with the onBell event. From there ppl can hook in whatever they want.

For the sound-addon I need a SoundContext anyway (thats the expensive and limited resource), thus think offering an onBell handler along with the other stuff does not hurt (and its rather small in code size).

[Tyriar]

Pushed to v5 branch