Add spiral gradients to the gradient tool

This commit adds Spiral and Reverse Spiral gradients to the gradient tool. Done by Miguel Lopez (Reptorian) @miguell .

Test Plan

Select the gradient tool, go to Tool Options, select Spiral or Reverse Spiral and paint with it.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.