Patch and warn late event registrations

[evnchn]

Motivation

Reading #4154 and #4828 again, I come to the following conclusion:

1. Late event registrations are fundamentally misaligned with Vue's design. (reference: Events such as keydown and keyup are not triggered by an ui.input when not assigned directly upon creation #4154 (comment))
2. Therefore, we do not need to aim for perfect behaviour if late event registrations do occur (so Force Vue re-render on late event registration #4828 (comment) should not matter)
3. Rather, "best effort patch" and "warning to the user" is the way to go.

Hence this PR.

Implementation

Code is extremely similar to #4828

Before we process the list of elements, we do a check to see if there are any new events sent in by the server.

• If there is, then:
  • We remove that element from Vue's this.elements
  • We mark that we need to await this.$nextTick();
  • We proceed to await this.$nextTick(); to force the Vue to render without that element
    • Do it ONCE only, for performance
  • In such a way, when we process the list of elements as usual, Vue will be like
    • "hey I haven't seen this element before, let me render it"
• If there is none, then fine, do nothing.

But on top, I added warnings both in Python-land and JS-land.

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• Pytests have been added.
• Documentation is not necessary (warning message self-explanatory).

Final notes

How I read it, #4828 was closed because:

• Accidental closure
• Significant merge conflict due to NiceGUI 3.0
• Focus on perfect behaviour (no focus drops)

Both of which should no longer be blockers, given the new code and new positioning on the issue.

So hopefully this gets a pass. Thanks!