NumericWidget : Mouse wheel adjust value

[ericmehl]

This adds the ability to change a numeric widget value with the mouse wheel in the same manner as the keyboard up and down keys currently do.

I arrived at the scale factor of 15.0 to translate the wheelRotation value to increment units by seeing what the wheelRotation value was for a single "tick" on my mouse. I don't know how universal that is, so could definitely do with some testing by others to see if it changes values in a reasonable way.

Fixes #6009.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.

[murraystevenson]

Thanks Eric! The scale factor of 15 feels about right for my Logitech mouse on Linux, I see roughly 1 (+/- 0.125) increment's worth of change for each "step" of the wheel. I've added a couple of thoughts inline based on some early testing.

[ericmehl]

Thanks @murraystevenson and @johnhaddon! I addressed the comments and also tested on a laptop track pad. It may be a touch too fast using a track pad, but I was still able to increment one at a time without much trouble. Since the mouse case feels natural, I'm inclined to leave it at the current rate.

[ericmehl]

The latest push refines the increment logic and also fixes the scroll signal leaking. In cb7e934 I added to the tooltip to try and make this a little more discoverable.

Ready for more testing and review!

[ericmehl]

I fixed the wording of the tooltip and squashed everything down to one commit. Hopefully ready to merge!

[murraystevenson]

Thanks Eric! Merging.