Implemented very rough management of layers

- Refactored the code that changes the current layer:
  xo_layer_active_set(gint layerIndex). This was mainly because the
  combo box needs to be set to an integer that is counter intuitive to
  the layer index. It also separates interface from logic.
- Added a flag ui.display_layers_above (defaults to false)
- Added an undo operation ITEM_MOVE_LAYER_DOWN
- Added the page number to the undo structure (since I need to know the
  page number)
- Added a new menu function: Move Layer Down. This feature will move the
  current layer one down, and will make that layer the active one. You
  cannot move layer 1 behind the background), and of course, you cannot
  move the background.