Placement above staff / below staff.

Extends supports for element placement above / below staff for:

*) ElementLayout
*) BSymbol, FSymbol, Symbol
*) Text, SimpleText, TextStyle, TextStyleData (with derived classes & relevant dlg boxes)
*) inspector
*) updated definitions of built-in default styles.

Should cover the majority of cases / element types. I expect some details will need more fine tuning. Should also ensure a substantial backward compatibility.

For a discussion see: http://dev-list.musescore.org/Elements-below-and-above-the-staff-td7577993.html