KCalendarSystem: Add support for alternative Week Number Systems

KDE currently only supports the ISO Week Number system used by most of
the world.  However some countries such as the USA use other week number
systems.  These systems are defined and supported by POSIX, Unicode,
Windows, and Mac so it's probably a good idea for us to support them
too.

Note that the old weekNumber() api will always return the ISO Week Number
for backwards compatability, but is now deprecated.  The new week() api
will return the localized Week Number and should replace all calls to
weekNumber().  If an app always requires the ISO Week then the week()
call can take an optional parameter to set this.

Supported systems are:

ISO Week: Current ISO week support.  The value of Week Start Day is
ignored.

First Full Week: Week 1 starts on the Week Start Day set in KLocale,
any days before this day fall in the last week of the previous year

First Partial Week: Week 1 starts on Jan 1 with Week 2 starting on the
Week Start Day in Klocale, so Week 1 may have less than 7 days.

Simple Week: Week 1 starts on Jan 1 with a new week starting every 7
days thereafter.  The value of Week Start Day is ignored.

Other systems may be added if required.

FEATURE: 224351
FIXED-IN: 4.7.0