Introduced a new syntax for accessing system-data

To keep backwards-compatibility I didn't remove the old system-date-formatting.
So there are two methods to access system-dates right now:

**1) $system:variable**
Used for accessing the entry-id and system-dates "the old way"
 - $system:time
 - $system:date
 - $system:year
 - $system:id
 - ...

**2) $system:variable:qualifier**
Used as "the new syntax" to access system-dates. Follows the qualifier-concept used for the field-data and allows using all qualifiers for php date_format - e.g:
 - $system:date:d
 - $system:date:Y
 - $system:date:H:i
 - ...