Fix background painting in the predefined brush selector
The patch does multiple fixes:
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Removes a hacky conversion of a PNG brush into a mask brush if the brush is all gray. All-gray brushes can easily be used in lightness mask mode.
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Implements separator lines in the predefined brush selector and fixes hidpi support in it.
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Implements checkers background painting for the brushes that support transparency, and therefore, lightness mode.
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All other brushes are now painted on white background
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To let the selector know if the brush has transparency, this property is written into the brush'es metadat. To do that, a special class KisBrushTypeMetaDataFixup is implemented. It updates the database, if finds any inconsistent reconrds at the database.