Support text elements for PySide widgets that display text

[berendkleinhaneveld]

Summary

• implement create_text_element() and set_element_text() in PySideRenderer with a lightweight TextElementProxy that holds text content and a weakref to its parent widget
• register insert/remove handlers for QLabel and QAbstractButton (covers QPushButton, QRadioButton, QCheckBox, QToolButton) that keep an ordered, anchor-aware list of proxies on the parent and display the joined content through setText()
• update the counter example to use child text instead of text attributes

This makes templates like the following work:

<widget>
  <label>Count: {{ count }}</label>
  <button @clicked="bump">bump</button>
</widget>

The approach follows #181 (pygfx text elements), but instead of type checks in the renderer's generic insert()/remove() paths, support is opted into per widget type through the existing register_insert/register_remove machinery. This keeps the generic paths free of feature-specific branches, makes it explicit which widgets support child text, and lets custom widgets add support the same way (see pyside/objects/textelement.py). Inserting text into a widget without registered support logs an error, like other unsupported inserts.

Note: since the proxies sync their joined content through setText(), child text overwrites a text="..." attribute set on the same widget — last writer wins.

Fixes #189

Test plan

• new tests in tests/pyside/test_pyside_text_elements.py: direct renderer tests (label/button text, updates, removal restoring empty text, anchor ordering, unsupported parent) and SFC integration tests (reactive updates via button click, v-if unmount clearing text)
• full test suite passes (366 passed, 1 skipped)
• examples/pyside/counter.cgx verified end-to-end offscreen: renders "Count: 0", click updates to "Count: 1"

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