Allow creating Animation method call key on Invalid Track path

[AxelSevenS]

In Godot, when you try to create a Method Call key on a Track that has a Path that is not currently valid, the error "Track path is invalid, so can't add a method key." pops up and you just cannot add a Method Call Key.

If you need to call a method on a Node that does not exist in the editor, for example, in a dynamic state-machine context, this is quite a problem, because that means you have to create the Node and put it at the path the Animation Key will use.

Fortunately, you can work around this issue by creating a Method Call track that has a correct path, choosing any method to be called, manually replacing the method name and arguments and switching the Track path.

I feel like this is kind of an accidental "hidden feature" because it is useful and it doesn't cause any errors if it can't find the path.

I would argue that instead of just popping the error message with a single "OK" button, it should give you the option to create an empty Method Call track that we can fill out manually.
For example:

Warning!
Track path is invalid, can't find any method to add.

• OK
• Add empty Key anyway