Edges where taskbar can be placed (in coherence mode).

enum _PRL_WINDOW_EDGE_MASK {
  WINDOW_LEFT_EDGE_MASK = (0x1L << 0),
  WINDOW_TOP_EDGE_MASK = (0x1L << 1),
  WINDOW_RIGHT_EDGE_MASK = (0x1L << 2),
  WINDOW_BOTTOM_EDGE_MASK = (0x1L << 3),
  WINDOW_ALL_EDGE_MASK = WINDOW_LEFT_EDGE_MASK|WINDOW_TOP_EDGE_MASK|WINDOW_RIGHT_EDGE_MASK|WINDOW_BOTTOM_EDGE_MASK
};