This PR adds first layout component: AlignComponent, an equivalent of Align widget in Flutter.
AlignComponent sizes itself to its parent, and then keeps its child aligned to the specified anchor within its own bounding box.
Also adding onParentResize() lifecycle method, which is similar to onGameResize, but fires whenever the parent of the current component changes its size for any reason. (FlameGame is assumed to have the size canvasSize, and will invoke onParentResize whenever the canvas size changes).
Additional layout components are planned to be added in future PRs.
