Fix error with abbreviated weekdays names for French calendar

**Issue example**:
- Configure a tree to use French calendar conversion
- Create a custom translation to display the weekday name in the dates (e.g. `'%j %F %Y' => '%D %j %F %Y'`)
- Open the calendar to date 13/01/1794 which is Nonidi 24 Nivôse II

**Error displayed / Stacktrace**:
`InvalidArgumentException: 8 in [...]\library\WT\Date\Calendar.php on line 417`

Function | Location
-------- | --------
`WT_Date->display( $url = ???, $date_format = ???, $convert_calendars = ??? )` | ..\calendar.php:141
`WT_Date_Calendar->format( $format = '%D %j %F %Y', $qualifier = NULL )`	 | ..\Date.php:218
`WT_Date_Calendar->formatShortWeekday( )`	 | ..\Calendar.php:649
`WT_Date_Calendar::dayNamesAbbreviated( $day_number = 8 )` | ..\Calendar.php:743`

**Cause**: 
The underlying cause is the 10-days weeks in the French Calendar, hence the expression `$this->minJD % static::DAYS_IN_WEEK`(line 743 of Calendar.php) can return a number superior to 6 (up to 9).
The problem  is the use of the `private` modifier in the base class that prevents the method `dayNamesAbbreviated` from being inherited/overriden by the `WT_Date_French' subclass implementation of the same method.

A change of the modifier from `private` to `protected` fix the issue. I have as well modified the other uses of the `private`modifier in the Calendar classes, just in case they need to be overriden by subclasses