Create EventSource at the right moment

[toy]

Closes #180

On small dashboard setTimeout with timeout 0 was enough to create
EventSource after all widgets were initialised, but on big dashboards
the ready event of layout (View instance for document) is the only event
that is happening after all widgets get initialised and registered in
the list.

[kinow]

@toy have you tested with 160 widgets and with long names too? I think for #181 the length of the generated URL wouldn't be fixed with only this change.

[kinow]

Confirmed it fixes #180 , not sure about #181 (URL length), so leaving that one open for now. Preparing a release with this PR, thanks again @toy!

[kinow]

Added changelog, pushed to master with the commit merged 👍

[toy]

@toy have you tested with 160 widgets and with long names too? I think for #181 the length of the generated URL wouldn't be fixed with only this change.

Locally (in docker image), so using thin, it works with 200 widgets each having id of 44 characters (longest in the issue was 42) resulting in a query string of 9401. Which confirms that all depends on the server and proxies on the way. But if one of them doesn't handle url or query string that long, the error should be very explicit with status 414 and no widget getting any data.
Apparently it is not possible to get the status on error event for EventSource, so no way to automatically switch to not sending ids.