Add Document::setBackgroundColor API

This can be used like this:

from krita import Krita
from PyQt5.QtGui import QColor

kt = Krita.instance()
doc = kt.activeDocument()

test_color = QColor(128, 128, 128, 255) # change as needed

old_background = doc.backGroundColor()
print("Old background: {}".format(old_background.getRgb()))

doc.setBackGroundColor(test_color)

new_background = doc.backGroundColor()
print("New background {}".format(new_background.getRgb()))

Patch by Jeroen Hoolmans, thanks!
fferential Revision: https://phabricator.kde.org/D12729