Add the possibility to change the color of individual events

[aubincleme]

The goal of this issue is to add the possibility to edit the color of individual events within a calendar.

Right now, colors are defined at the level of a calendar, however it can happen that some events end up being more important than others, and thus need to be highlighted with a different color.

The current functional solution we give to users of the application is to create two calendars, each with a different color, and then use the mocca calendar macro to display the two calendars in the same view.