Properly determine the game when cloning instances

[DasSkelett]

Problem

A user reported a problem when trying to clone a KSP instance, they're getting a Clone failed: Object reference not set to an instance of an object error.
We couldn't quite figure out what the user was doing exactly, the messages were kinda vague:

I remember I was messing around with the specific path, and I changed it, but the changes didn't show up, so I figured it hadn't saved my changes or something

But after looking at the clone UI logic, these lines stood out as a possible cause for a NRE:

CKAN/GUI/Dialogs/CloneFakeGameDialog.cs

Lines 149 to 156 in 2564dfb

	GameInstance sourceInstance = manager.Instances.Values
	.FirstOrDefault(i => i.GameDir() == textBoxClonePath.Text);
	GameInstance instanceToClone = new GameInstance(
	sourceInstance.game,
	textBoxClonePath.Text,
	"irrelevant",
	user
	);

They were refactored in #3223 and didn't account for the possibility of a path being entered that didn't already belong to a known instance, which would result in a null sourceInstance.

Changes

The underlying issue that forced this instance lookup was that we didn't have a good way to determine the game of the instance to clone.
In GameInstanceManager.AddInstance() we already had a basic layout for this, but it was missing the option to prompt the user to choose between multiple games if it detected multiple potential ones.

This logic is now moved into a new DetermineGame() method, which calls User.RaiseDelectionDialog in the aforementioned case.

Both the CloneFake dialog and the AddInstance button use the new DetermineGame() method.

For testing you can simply copy KerbalSpaceProgram.cs, rename the file and class, and add it to GameInstanceManager.knownGames to force multiple games to be detected.

[DasSkelett]

Ah, that's why it's passed to AddInstance(). So going to pass it to DetermineGame() as well, and try to leave a comment for the User field, maybe it helps the next time.

[DasSkelett]

I just got the idea that we could reassign the User property of the GameInstanceManager at start-up of the GUI instead, and eliminate the problem once and for all.
Do you see any pitfall that we could hit when we do this?

[HebaruSan]

One more update, it's weird that the instance list shows that we know each instance's game, but then when we go to clone them we may prompt the user. Pushed a commit to make the prompting a fallback if we can't get the game from the instance (i.e., if the paths don't match).

[HebaruSan]

This looks good to me now, but in case you want to make some more changes, I'll wait a couple of days before merging it.