Adding support for re-ordering scenes

Sorry, this PR is waaay to big but it was originally split in two and did not want to rebase twice. Also I had to do several things and also fixed/improved stuff I found on my way.

This PR basically adds:

Support for "notes". A note is a basic annotation component that has a title and a description. Now gamemetadata has a note instead of a title+description. Scenemetadata also has a note. In the future, both components (and any others) may have a list of notes instead of just one
Support for scene metadata. I still feel that too much code had to be duplicated for this (see jtorrente@1b2e10f).
Support for renaming scenes

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).