feat: add UI.triggerAfter for deferred server-side callbacks

[Artur-]

Add UI.triggerAfter(Duration, SerializableRunnable), which runs a task on the
server once the given delay has elapsed. The delay is measured by the browser
via a one-shot client timer that makes a normal round trip when it elapses, so
a single deferred event can be handled without enabling push.

This change:

• adds UI.triggerAfter, a facade that wires the trigger and action and returns
  a Registration; removing it clears the client timer, so a task cancelled
  before its delay elapses does not run;
• adds TimeoutTrigger, which arms a one-shot setTimeout on the host element and
  clears it on removal;
• extends CallbackAction with a value-less constructor (SerializableRunnable)
  that forwards no value and renders $0(), used by triggerAfter.

[github-actions]

Test Results

 1 442 files  +3   1 442 suites  +3   1h 24m 10s ⏱️ + 1m 55s
10 123 tests +6  10 055 ✅ +6  68 💤 ±0  0 ❌ ±0 
10 595 runs  +6  10 526 ✅ +6  69 💤 ±0  0 ❌ ±0 

Results for commit 6765fea. ± Comparison against base commit 979e73a.

♻️ This comment has been updated with latest results.

[Artur-]

Use case this this feature would be at least:

1. Temporarily changing a text in the UI and then changing it back, e.g. Changing a "Copy" button to "Copied" and then 1s later back to "Copy":

button.setText("Copied"); UI.getCurrentOrThrow().runAfter(Duration.ofSeconds(1), () -> button.setText("Copy")})

3. Delaying initialization of a view or part of a view

// 0 still forces a client round trip
UI.getCurrentOrThrow().runAfter(Duration.ofSeconds(0), () -> { doHeavyInit();})

[Legioth]

The core of the implementation looks sensible but I have to prioritize other work this week so I won't do a thorough review of the tests and the documentation.

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud