Set up KMM Project Structure

[westnordost]

Currently, the project setup is that of a native Android app. The task is to

1. change the project structure to that of a Kotlin Multiplatform Mobile project
2. move all files that are already pure kotlin at the time this task is done (no Java or Android dependencies) into the common module, including its tests. (The implementation for iOS is obviously empty for now)
3. test build on Android

After this, work can start on a skeleton implementation of the iOS app.

This should ideally be done when there are as few PRs open as possible, as all PRs will result to have conflicts in every single file.

This should also ideally be done in concert with @Helium314 (the maintainer of the SCEE fork/addon), i.e. the fewer unmerged branches there are, the less work it is to rebase it on top / merge it with all the moved files.

[HolgerJeromin]

git is able to track renames in rebases. But yes: It can not do magic.
Rename files and change them (to match its new location) should be done in separate commits.