Added background color for markers

[vallsv]

Related to #4005 #4010

Previously an automatic background color was added to the marker text of the OpenGL backend to try to improve the readability of the text. That's unfortunately not enough, so better to let it open to the designer.

• Now the background is used for both backend
• And designer can specify it as marker.setBackgroundColor("red")
• The automatic background was dropped

        label = Marker()
        label.setPosition(50, 50)
        label.setText("Foo bar\nmmmmmmmmmmmmmmmmmmmm")
        label.setBackgroundColor("#FFFFFF44")
        plot.addItem(label)

        label2 = Marker()
        label2.setPosition(70, 70)
        label2.setText("Foo bar")
        label2.setColor("red")
        label2.setBackgroundColor("#00000044")
        plot.addItem(label2)

        label3 = Marker()
        label3.setPosition(10, 70)
        label3.setText("Pioupiou")
        label3.setColor("yellow")
        label3.setBackgroundColor("#000000")
        plot.addItem(label3)

Changelog:

• Markers now support a text background color
• The automatic text marker background was dropped for the opengl backend

[vallsv]

Here is fixes as requested.