fix: Change DialogueView to a mixin class

[projectitis]

Description

The DialogueView is responsible for handling and (almost always) displaying the dialog to the user. A very common use case requires the DialogueView to be a PositionComponent or other visual component in the game. This would require extending from both PositionComponent and DialogueView.

Problem: The current DialogueView implementation is an abstract class. Dart does not support multiple inheritance. This means that the case above is difficult to implement.

Solution: Changing the DialogueView from an abstract class to a mixin class allows the developer to extend PositionComponent and mix in DialogueView. The class can now be used as a base class OR as a mixin. All existing tests still pass. An additional test has been added that validates the use as a mixin (extending one class and mixing in DialogueView).

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.

Notes on docs: Updated class docs. Checked doc pages. These still read correctly.

Notes on examples: Currently no jenny examples.

Breaking Change?

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

Related Issues

NA

[projectitis]

Ah, didn't know that was possible, thanks @spydon. Tests still pass, too.

[spydon]

Lgtm, thanks for your contribution! :)