Feature 75; added a preview window

https://www.keepassx.org/dev/issues/75
I've added a preview button and window which generates 20 keys in a
window created for that purpose. It has a progress bar, and a close
button.

I'm a bit new to this so if I'm open to any critiques, suggestions,
tips, etc.

Third times the charm ;) Hopefully this one works without a hitch.