Adding support for re-ordering scenes

This PR needs #132 first. It provides basic support for reordering scenes. To test it, right click on the scene widget to display a context menu and click up or down.

May look simple but it got a bit complicated because I created a reusable action for reordering things that I consider quite neat.

@anserran, you may want to have a look at Reorder and use it to change the Z of scene elements (#122).