Automatically determine the first weekday for a calendar control

If neither wxCAL_SUNDAY_FIRST or wxCAL_MONDAY_FIRST was given, use
wxDateTime::GetFirstWeekDay() to automatically determine the preferred
day.

This changes the earlier default behaviour, which was to use Sunday
if not otherwise specified. However, the wxGTK native calendar control
ignored this behaviour anyway.

src/common/calctrlcmn.cpp

@@ -195,5 +195,26 @@ bool wxCalendarCtrlBase::SetHolidayAttrs()
     return true;
 }
 
+bool wxCalendarCtrlBase::WeekStartsOnMonday() const
+{
+    if ( HasFlag(wxCAL_MONDAY_FIRST) )
+    {
+        return true;
+    }
+    else if ( HasFlag(wxCAL_SUNDAY_FIRST) )
+    {
+        return false;
+    }
+    else
+    {
+        // Neither flag was explicitly given, let's make a best guess
+        // based on locale and/or OS settings.
+
+        wxDateTime::WeekDay firstDay;
+        wxDateTime::GetFirstWeekDay(&firstDay);
+        return firstDay == wxDateTime::Mon;
+    }
+}
+
 #endif // wxUSE_CALENDARCTRL