Handle artboards with the same name

[nikoladev]

If artboards have the exact same name the last one will be the one that's exported (more specifically, it will overwrite the previous one(s)).

Possible implementation:

1. Keep a list of the current names
2. If a name occurs in the list add -2, -3, -4, etc. to the end of the filename

To keep in mind:

1. How will this interact with Ask user if they want to overwrite #1?
   • Do the renaming first and then check for overwriting?
   • OR:
     1. If they don't want to overwrite: check for overwriting, do the renaming, then check for overwriting again
     2. If they do want to overwrite: do the renaming and don't worry about overwriting