[API] Allow null to be passed as the shortcut param to MenuItem

[sh54]

Fixes #34

I don't know JNI too well so I think my fix could be less verbose. All it really needs to do is communicate to the autogenerator that the shortcut param can be null. Not checked in any updated binaries.

[SpaiR]

Hi. Thanks for your contribution!
The solution you've applied is indeed a little bit verbose... What you actually need to do is something like that: https://github.com/SpaiR/imgui-java/blob/master/imgui-binding/src/main/java/imgui/ImGui.java#L788-L796

[sh54]

Thanks for the feedback. I feel a bit more comfortable with the JNI system now. I had a look through the different bindings to find an applicable pattern but I missed the setWindowFocus implementation.

I pushed a new version that follows that example. I also altered overloads like menuItem(String label, String shortcut) to call the custom bindings since they will otherwise throw on a null shortcut.

[SpaiR]

Looks good! I was planning to do a release in the near future, so those changes become available as well.