Move vala implementation into vala sub-directory, we'll go back to python.
Lots of improvements in python version: code refactor, new features

- New monochrome icon taken from Gnome
- Gnote support, will use any available service
- Notebook filter support
- Last modification filter support, today, less than 7/30 days / 1 year
- Handle note events (refresh on new/removed note, update notebooks, etc.)
Move back python stuff at project's root
Preparing for packaging, fixing some paths...
Remove deprecated on_active method