Add color widget
It may be useful to allow user to enter easily an accurate color. For that we have https://docs.gtk.org/gtk3/class.ColorButton.html and https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/color available. As the common representation is RGB in hexadecimal, we could use that as basis. The widget will be based on char field and should be lazy about the content as the widget is only an helper to enter the value. The developer how wants to enforce the content can do it with a validation.
The widget should be available for form and dict.
I do not think it is necessary for editable tree view as this is any way a slow input method which is the opposite of goal of editable tree view.
Related merge requests 1
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