feat!: HasDraggableComponents mixin is no longer needed

[st-pasha]

Description

This PR is first in a series of refactors that aim to simplify event handling in Flame. The approach is as follows:

• Added class GestureDetectorBuilder, which encapsulates the logic of applyGestureDetectors() in a class. This class resides within the Game and initiates widget rebuild whenever any new gesture detectors are added or removed. Note: [idea] Convert HasTappableComponents/HasDraggableComponents into classes #1733 suggests having a list of interfaces inside the Game class -- this is essentially that list, encapsulated in a class.
• Added the MultiDragDispatcher component, which contains the logic that used to be within the HasDraggableComponents mixin. This component is internal; it mounts to a FlameGame directly, and ensures that it is a singleton.
• Whenever any DragCallbacks component is added to a game, it automatically adds the MultiDragDispatcher component (unless there is already one), which in turn registers a drag gesture detector with GestureDetectorBuilder and rebuilds the game widget.

The end result is that now in order to make a component draggable you only need to add the DragCallbacks mixin to that component, everything else will be handled by the framework.

Consequently, the HasDraggableComponents mixin is now empty and marked as deprecated.

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.

Migration instructions

The HasDraggableComponents mixin is now empty & deprecated. If your game used this mixin overriding its methods onDragStart, onDragUpdate, etc -- then they will no longer work. If you want to receive drag events at the top level of the game, then simply add a DragCallbacks component to the top level of the game.

Related Issues

WIP for #1733

[spydon]

Great step forward for the event system!
We shouldn't do any release until tappable etc also is done to keep consistency.

[luanpotter]

Nice!