Added better gestion of the classes added in wordpress menu editor

[xposedbones]

In the previous code, the user was not able to add multiple class to his nav item. The code would always pick the first class. With my changes, the user is able to add as many class as he wants to the nav item without any issue.

[roryashfordbentley]

I'll try to find some time to test this out thoroughly but I see no reason not to implement this. Great job!