The invalidate_style() calls on text nodes in did_receive_focus() and
did_lose_focus() were no-ops since Node::invalidate_style() returns
early for character data nodes. Replace them with set_needs_repaint()
which properly invalidates the containing PaintableWithLines' paint
cache, ensuring selection highlights are cleared and the caret is
repainted when switching focus between text controls.
Fixes#8363
