[ufrshubham]

Description

GameWidget picked its focus node once, in initState, and never looked at focusNode again. When
the same GameWidget was rebuilt with a different focusNode, the new node was never attached, so
requestFocus() on it did nothing and the game stopped receiving key events. dispose also decided
whether it owned the node by checking widget.focusNode == null at that moment, so after switching
from a user node to no node it disposed the user's node, and after switching the other way it never
disposed its internal node.

Now GameWidgetState:

• Keeps the node it creates in _internalFocusNode and uses widget.focusNode ?? _internalFocusNode,
  so a new focusNode is used as soon as it is passed.
• In didUpdateWidget, when focusNode changes and the old node has the primary focus, requests the
  focus on the new one. If a descendant, like an overlay with a text field, has the focus, it is left
  alone, since it is moved under the new node in the next build and keeps the focus.
• In dispose, only disposes _internalFocusNode.

Tests in game_widget_keyboard_test.dart cover a new node taking over the focus and key events,
switching between a user node and the internal one, a user node not being disposed, and an overlay
keeping the focus when the node changes. The focusNode dartdoc and the keyboard input docs
describe the new behavior.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [-] I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

Closes #4159