[Feature] Launch Game button in the Mod Manager

[RyanWalpole]

Overview:
Add a button that allows the user to launch the game straight from the mod manager. This would come with some possible roadblocks that would need to be considered:

• Launch button must be disabled if game is already running
• Program must be able to tell that the game is running even after it has first decided
• If the SMAPI exe cannot be found, default to regular exe

Updating the Button
Button will have to check periodically if there is a Stardew Valley instance running as well as a SMAPI instance, determining the state of the button.