Add Qt button to switch to Lite mode.

This pull includes an new icon and puts a button, next to the password button, that allows switching instantly to Lite mode. The main advantage is that it makes it easy to toggle back and forth between modes. 

I'd suggest this pull actually gets tested a fair bit on Windows and with more full use. I did some testing but only on Ubuntu and without sending a payment. It worked fine for me but I'm a bit dubious about creating the Lite mode window and may possibly have missed something important. 

Expert eyes please review. 

Also, I'm sure someone can come up with a better icon than the one I included.