Add opacity hover tooltip

[Frankenlist]

Resolve #149

[claviska]

Thanks for contributing this. A couple things I noticed:

• The tooltip disappears when you drag fast or move the mouse off the handle. We should probably activate it on some type of start drag and deactivate it on end drag instead.
• The font is a bit small. I think it would be fine if it was larger and even had more padding — maybe even move the tooltip above the cursor instead of on top of the hue selector.