Add Qt properties for plot widget colors

[vallsv]

This PR expose PlotWidget colors as Qt properties. That's a low cost addition to be able to custom style using qss syntax

Here is an example to edit the style of the whole plots.

styleSheet = """
PlotWidget {
    qproperty-backgroundColor: #19232d;
    qproperty-dataBackgroundColor: #19232d;
    qproperty-foregroundColor: #B0B0B0;
}"""
qapp.setStyleSheet(styleSheet)

Here we can see that the plot from the colormap dialog was also patched.

Changelog:

• Expose PlotWidget colors as Qt properties