Added a QLabel for color preview to KisHexColorInput

According to https://krita-artists.org/t/specific-color-selector-color-preview-idea/14461 it would be nice to see the color when using the specific color selector. I added it to KisHexColorInput::KisHexColorInput via a QLabel and update the color via setStyleSheet in the update method. Not sure if that was the right place for it but it was the only way I could figure out how to place the label directly beside the input box. I believe it is working correctly with the specified color models RGB, CMYK, etc.

Test Plan

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