Update vpltk module based on QtPy

[pradal]

Plantgl (https://github.com/openalea/plantgl) has copied and modified vpltk.
But modifications had not been back propagated inside vpltk.
Update vpltk according to https://github.com/fredboudon/plantgl/blob/master/src/openalea/plantgl/gui/qt/

@Wilgawox will do the modifications reviewed by @fredboudon

[pradal]

Update the code of vpltk with latest modifications from plantgl

[pradal]

This will be difficult because the logic depends on QT C++ version used to build plantgl.

The most robust strategy would be to depend on QtPy