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.

interface/wx/calctrl.h

@@ -8,7 +8,7 @@
 enum
 {
     // show Sunday as the first day of the week (default)
-    wxCAL_SUNDAY_FIRST               = 0x0000,
+    wxCAL_SUNDAY_FIRST               = 0x0080,
 
     // show Monday as the first day of the week
     wxCAL_MONDAY_FIRST               = 0x0001,
@@ -254,6 +254,11 @@ enum wxCalendarHitTestResult
     month is changed, so you will often want to update them in
     @c EVT_CALENDAR_PAGE_CHANGED event handler.
 
+    If neither the @c wxCAL_SUNDAY_FIRST or @c wxCAL_MONDAY_FIRST style is given,
+    the first day of the week is determined from operating system's settings,
+    if possible. The native wxGTK calendar chooses the first weekday based on
+    locale, and these styles have no effect on it.
+
     @beginStyleTable
     @style{wxCAL_SUNDAY_FIRST}
            Show Sunday as the first day in the week (not in wxGTK)