Add brush fade python api

Aqaao Aqaaorequested to merge
aqaao/krita:fade-interface into master

Added brush fade set/get python interface.

Test script 

view = Krita.instance().activeWindow().activeView()
fade = view.brushFade()
print(f"Current fade:{fade}")
view.setBrushFade(0.1)
fade = view.brushFade()
print(f"Current fade:{fade}")

Running above script in Scripter will allow you to observe the brush fade changes.

(brushFade() method will always return 1.00 for brushes that cannot adjust fade.)

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.
  • Does the patch add a user-visible feature? If yes, is there a documentation MR ready for it at Krita Documentation Repository?

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