[FIX] owpaintdata: Adjust color model to input dataset

[ales-erjavec]

Issue

Fix a IndexError when the input dataset's class variable has more then 17 values.

Traceback (most recent call last):
  File "/Users/aleserjavec/workspace/orange3/Orange/canvas/scheme/widgetsscheme.py", line 822, in process_signals_for_widget
    handler(*args)
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/data/owpaintdata.py", line 1029, in set_data
    self.reset_to_input()
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/data/owpaintdata.py", line 1051, in reset_to_input
    self._replot()
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/data/owpaintdata.py", line 1230, in _replot
    pens = [pen(self.colors[i]) for i in range(nclasses)]
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/data/owpaintdata.py", line 1230, in <listcomp>
    pens = [pen(self.colors[i]) for i in range(nclasses)]
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/utils/colorpalette.py", line 657, in __getitem__
    return QColor(*self.getRGB(value))
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/utils/colorpalette.py", line 667, in getRGB
    return self.rgb_array[-1 if np.isnan(value) else int(value)]
IndexError: index 18 is out of bounds for axis 0 with size 18

Description of changes

Adjust the color palette to the input dataset's class var colors.

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

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