Added getSeparatorStyle() and setSeparatorStyle() member functions for FXSeparator.

The FXHorizontalSeparator and FXVerticalSeparator classes were refactored slightly, introducing a new intermediate base class, FXSeparator. Now, FXSeparator subclasses FXFrame, and then both FXHorizontalSeparator and FXVerticalSeparator subclass FXSeparator.