User Story

As a developper, I want to set useful colors to indicate the content, status, or other attribute of the CalendarItems in my calendar.

Currently it is possible to set HeaderColor and ContentColor, but not font color (for header or content).

This means that it is not possible to choose a good contrasting color to match the background, and cause the calendar to be unreadable if the background colors are too close to the default font color.

Acceptance Criteria

https://github.com/zkoss/zkcalendar/blob/master/calendar/src/org/zkoss/calendar/api/CalendarItem.java

has a getHeaderFontColor, getContentFontColor attributes, 

can be set in SimpleCalendarItem when creating a new item

will show the relevant font color at client-side.

Details

Need to update both the calendarItem classes, the calendar Utils at server side (to transfer the value) and the client-side JS functions that use these values to generate the item html code.