Move game selection to a modal within the main window

[cohaereo]

With the renderer being separated from the main app in 0.6, we can embed the game selection window into the main window, also allowing for the package manager to be initialized asynchronously (eg. don't block the whole GUI while waiting for it to initialize)

Might be able to use the new egui 0.30 Modal API for this
https://docs.rs/egui/latest/egui/containers/modal/struct.Modal.html#method.show