You now have to pass an Orientation to the GSlider constructor. It's not
possible to change the orientation after construction.
Added some vertical GSliders to the WidgetGallery demo for testing. :^)
Regardless of mode, made the knob container clickable so the knob position
can be moved without dragging the knob itself.
Added a 3rd GSlider to the WidgetGallery showing the proportional mode in
action.
It's good to have a place where we can try out all the different widgets.
This needs some more work on a nice layout, and should also include more
of the widgets. :^)
