Epic: merging xplat-editor work into master

Work on the xplat Avalonia-based version of the Game Studio started some years ago. It is in various states of progress in the xplat-editor and other related branches (as well as some private branches on my private forks).

The original goal was to keep it separate from the main branches (such as master), but it has diverged too much and every time I merge master into it, I run into issues.

Instead, we will merge it back to master pieces by pieces. This will make it easier to continue the work, advertise it and enable more contributions from more people.

This epic tracks merging all that's be done so far. However it will not track the overall completion of the port.

• Renaming/moving legacy WPF projects: #3433
• Basic skeleton of all new projects
• Update core presentation projects
• ... (more to come)