Gradient Widgets Improvements (4.3)

port of !591 (merged) to 4.3

I encountered a problem that happens also with the global gradient chooser. If a gradient is edited it seems to be correctly updated, but when restarting Krita they look as they were before the editing, like if they weren't saved when Krita closed. I wasn't capable of fixing that.

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.