- make `UserActivity.InGame` and derive that to `InSoloGame` and `InMultiplayerGame`
- rename `SoloGame` to `InSoloGame`
- rename `MultiplayerGame` to `InMultiplayerGame`
As discussed in https://github.com/ppy/osu/issues/13864, Squirrel will
use the product name before the title, allowing us to use this variable
to update the icon while not changing the window display title or naming
elsewhere.