Main Menu: Mounts

[JJL772]

Description

Initially we didn't want to implement a mounting API on the main menu, but here we are... Beta testers are finding it difficult to mount games, even with the edit guides in mount.kv- we can't fault them for this either, eventually our game will be played by non-tech savvy people.

Long story short, we need a simple game mounts interface. This doesn't need to be anything big and grand, it can be as simple as a pop-up list of games with checkboxes next to them (kinda like gmod's menu).

This should be a component of the main menu, where to put it is anyone's choice.

Relies on #47

[vrad-exe]

I think it would be better to just make this a tab in settings, rather than having a dedicated menu like gmod does

[JJL772]

That works too.