Add the ability to mark notes and notebooks with color

[qumuq-til]

Currently the only way to find a note or a notebook is to read its name. I suggest there should be an option to mark notes in the note list and notebooks in the notebook list with different background colors so that they could be located and accessed faster.

In case it breaks Gnome guidelines (background color in the note list would interfere with the currently selected color highlight), color dots or, better yet, vertical color bars, on the right side of the notes list could be used.