Create LayoutComponent classes

[st-pasha]

Proposal

LayoutComponent will be the base class for a family of utility classes which handle the layout of their children. This would be quite similar to Flutter layout widgets such as Align, Column, Center, Container, Padding, SizedBox, etc.

The purpose of these new components is to assist with constructing UIs within a game: dialogue boxes, HUD, informational screens, in-game UIs such as dialogue or trade windows, etc.

PR #1971 already attempts to add two such layout classes: Row and Column -- this proposal is to have more of them, all under a single umbrella of LayoutComponent.

For example, suppose you're making a game and you want to display current score in the top right corner. Then, you should be able to declare it like this:

camera.viewport.add(
  Padding(
    padding: EdgeInsets.all(20),
    children: [
      Align(
        anchor: Anchor.topRight,
        child: ScoreComponent(),
      ),
    ],
  ),
);

After this, the game would take care to properly position the ScoreComponent and move it whenever the game resizes.

Implementation

LayoutComponent would rely on the onParentResize lifecycle method (#1421).

The tree structure can be either normal (the target is added as a child of the LayoutComponent), or flat (the target is added to the parent directly, and layout component is a sibling). I'm not sure which option is better here.

[spydon]

Sounds good! Should we close #1971 then?

[st-pasha]

Should we close #1971 then?

We'll add the new layout classes, and then #1971 could use this new interface to implement the Row and Column.