Editor scenemetadata

Sorry, this PR is probably larger than it should, but 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 https://github.com/jtorrente/ead/commit/1b2e10f5646c4b42d73e400b18999e8ff06a5a20).
- Support for renaming scenes.