Add workaround to properly display overflowing child elements of panes

The workaround preserves the initial 'auto' z-index of 'panes', so that
it is possible to keep many overflowing child elements of many 'panes'
visible in front of all other 'panes' while the mouse is neither over
the 'pane' or its children;

Actually, this solution replaces handing of z-indexes via:
allowOverflow( elem_or_pane )
resetOverflow( elem_or_pane )
showOverflowOnHover 
... in most of the standard use cases.