Improve split layer UI

[forman]

Expected behavior

Cesium allows layer split modes: show left or right side of a split position. The position applies to all layers.

The GUI should reflect this by making the split modes a property of the selected layer in the LAYERS panel and the position a view property that is changed globally. Whenever we have a layer split, we also show the splitter to change position.

Actual behavior

We just have a checkbox in the VIEWS panel. It is either not working correctly or its behavior is confusing.

Specifications

Cate Desktop 1.0 to 2.0-dev4