Modifications to Electrum to print wallets with Piper (not a real pull request)

Hi, I made a few small modifications to the classic electrum gui to add the ability to print keypairs and the wallet seed to paper using the printer inside Piper (https://piperwallet.com).  

This isn't really a pull request, I was just wondering if you could just glance over the modifications I made to gui_classic.py to tell me if I'm doing anything horribly wrong.  

I added two menu items to the Export menu and a menu item to the contextual (right click) menu when clicking on an address on the receive tab.  

Your code is really clean and easy to modify, so thanks for writing awesome code!