[TxMt] bug: numbering of untitled window really wonky

[Caio Chassot]

this seems to be the case since at least beta 5, and apparently still 
present. Suddenly the numbering of the new, untitled documents begins 
to jump, and so I'm at untitled 86, then it's 295, and it keeps on 
skiping numbers. (the numbers here are examples only)

I don't really know what triggers it, but my usage pattern is really 
simple: I mostly just load a lot of sketch projects from the terminal, 
and every now and then create a new, non-project file.