use wx.TheClipboard for copying features between two instances

I just implemented the use of wx.TheClipboard. This enables to store different types of content between applications.

So I made a text copy of the dna the standart. It gets pasted whenever someone copys from DNApy to gedit or somewhere else.
If you paste from DNApy to DNApy it will instead try to use a different content I  called "application/DNApy". It is the same object as the self.clipboard instance you used before.

The functions RichCopy and RichPaste handle the operations.

Take a look at the code, I think it is a rather small change to the previous state.

I now will try to make selection more powerfull, to allow selections with start>finish in circular dna.