Add ability to configure the colors used for displaying Meeting events

[slauriere]

When the Meeting Event Source is activated, Meeting events are currently displayed with a gray background and text in white. This task is about adding the ability to configure the colors to be used for displaying Meeting events. Since the Meeting event source can be activated either in a specific calendar or globally, the color settings should be configurable also at calendar level (with the option to use the same colors as the containing calendar) or globally (colors used for displaying such events in a calendar whose scope is the whole wiki).