Allow to override collection colors locally

[rfc2822]

Often the server doesn't send a color for a calendar / task list, or it sends a color that is not one desired by the user.

DAVx5 could allow to select / override the color of collections explicitly. These colors should then be used for the local calendars / task lists.

[rfc2822]

Note: There's a similar request for the title, see #1002