Improve the current tag-system

Right now tags aren't very useful. The only way to access tags is by a search.

I propose to:

 * Add a button somewhere on the left which switch the notebook-panel with a tag-panel
 * Here tags can be deleted, edited and clicking on a tag-title all notes with that tag are shown
 * Selecting more than one tag (on the left) should be possible (and then show all notes with one of the tag selected)
 * If you change the color of a tag, that tag under notes should change the color to match the one on the tag-panel
 * When clicking on a tag under a note should trigger the tag-panel (if not already shown) and show all notes with that tag

We should find a way to implement a filter to show notes with all tags selected.

This is just an idea, but we definitely should provide a list of tags (on the right, on the bottom, instead of notebooks, as a menu, I don't know) which can be hidden and clicking on tags should list only notes with that tag, like clicking on the tag itself.

We need a mockup from someone of the UX team.