Add sliders to the zoom view

[dendvz]

Channel offset, trigger level and trigger position sliders are attached to zoom view. First two slider sets behave as active mirrors of respective main sliders; any slider position change is immediately applied to its mirror. The trigger position slider in the zoom view is inactive, it reflects relative position of the trigger within zoom window.

Further enhancement could be adding 3rd view to DsoWidget:

• main scope & sliders only
• split view (the one we have now), both main and zoomed scopes are displayed
• zoom view (the new one), main scope & sliders are hidden.

[davidgraeff]

Nice. I have implemented the OpenGL 3 / OpenGL ES renderer. This does not conflict with your changes, but be careful to change anything inside of the GlScope or Exporter classes.