Add a self variable in the Variable view

If we consider the usual Variable view when debugging Java code, its content changes according to the selected element in the stack (Debug view)
Most notably, it has a **this** variable.

In our case, we could also have an equivalent concept and have a **self** variable. Its content would be the target of the selected element in the stack.

Note1: in the case of the concurrent engine. As a LogicalStep is composed of several calls, the **self** variable would be an array.

Note2: mixing RTD and fixed data in the same view can be handled by improving the UI. for example fixed data could use an *italic* font and RTD could use an icon overlay