Add socket_id to UIEventArguments; use new client.target where possible

[falkoschindler]

This PR addresses issue #4016 and feature request #4218:

• UI events should provide a socket ID to be used with the client.individual_target context.
• ui.navigate, ui.download() and ui.notify() should adhere to the client.individual_target context.
• ui.download() on the auto-index page (without using client.individual_target) should raise an exception.

Unfortunately, the current implementation has one main drawback:

• The new socket ID argument affects all existing event argument classes. Therefore this change might break existing use code.

The PR is still a draft because we're thinking about alternative solutions, like automatically calling event handlers in the context of the current socket. This would automatically trigger notifications, downloads and navigations on the socket that caused the event. But it could also affect run_method() and run_javascript() calls, which might not be desired.

[falkoschindler]

In f7d3fb7 I added a default '' to the socket_id field and removed it from the initializer. This way existing user code doesn't break when initializing event arguments without socket ID, and Python 3.8 and 3.9 don't complain about the order of positional and keyword arguments in derived classed. Given the limitations of dataclasses in Python <=3.9, I'm rather pleased with this solution.