Add Option to Hide Launcher on Game Launch -- Steam Deck Support

[serioustable]

Current Issue: On Steam Deck in Gaming Mode, Gamescope rapidly switches between the launcher window and the game window after the game is launched because it doesn't know which window to focus on; this is a common and known issue on any game that has a launcher that doesn't close.

Proposed Solution: add a "Hide Launcher on Launch" option to the Launcher settings so that once Veloren launches, the launcher is closed and Gamescope remains focused on only one window. If the launcher would re-open when Veloren is closed, that would also be beneficial, but optional.

Otherwise, the game runs very well!