Correct composite builds

[Goooler]

• After this, it could be run from the root project.
• AGP versions used by demo-app and the root project should be kept the same.
• includeBuild phrases should be placed in pluginManagement block.

[svenjacobs]

@Goooler Thanks for the pull request. Would it be possible now to specify the Reveal dependencies in demo-app via project references and not use the artifacts from Maven Central?

[Goooler]

We have to move demo-app out as a normal subproject instead of a standalone root project, or there will be a circular dependency issue in reverse.

[svenjacobs]

In commit afe6b08 I moved the demo app to a separate project. It was a subproject before. There was some reason why I did it but I don't remember. Maybe it was a build issue because of Kotlin Multiplatform, maybe it had something to do with the publication of the library. I don't know.

[Goooler]

Or we can let demo-app include build reveal-root, shared module now depends on core and shapes.

[svenjacobs]

Or we can let demo-app include build reveal-root, shared module now depends on core and shapes.

I think I like that better. But instead of the substitutes, couldn't we directly use project(":reveal:core") etc?

[Goooler]

I'm not sure how to depend on composite build subprojects instead of module notations.