[gui.glumpy] ensure that vbo upload happens in the right GL context
[gui.qt] support multiplots
[gui.qt] always fork in launch_qt_app

Otherwise we run into trouble when first creating a QApplication
in the main process and then trying to reuse it in a child process.
[gui.qt] add separate_colorbars argument to visualize_glumpy_patch
[demos] improve plotting in demo applications