[plot3d] Allow to set font used in axes labels

[t20100]

Idea would be to allow to change the font in use for the axes labels, i.e., allow to do the following from the public API:

font = sceneWidget.getSceneGroup()._getScenePrimitive().font
font.name = 'Arial', 'mono'
font.size = 15
font.weight = 74
font.italic = True