Slider y pos could control transparency

[jo-chemla]

Have the background map canvas always take 100% width/height, and add control similar to app.iconem.com media viewer (persp/ortho): the slider horizontal position would still control foreground canvas width, and the slider marker vertical position would control the foreground canvas opacity.

Possible way to do it: add to the class="gutter gutter-horizontal" component a vertical slider height:100%, which position controls foreground map transparency in Split mode only. Downside of this implementation is that selecting the handle and moving up/down and left/right at the same time might not be interpreted on both axes (unless eventPropagation activated correctly)