Make project a plugin instead

Move from uproject to uplugin to eventually make the project available for use in other projects.
This will require some work, even though most of the preparational work has already been done:
The UI structure has to be changed to make it easy to include it in the projects own UI. This might somewhat work already, however at the moment the position of all UI elements is too restricted.
The GameInstanceSubsystem has to be more customizable.