Add wxDateSpan::GetTotalMonths() method.

This is similar to the existing GetTotalDays() and counts both months and
years.

See #14704.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72615 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/datetime.h

@@ -1579,6 +1579,8 @@ public:
     int GetYears() const { return m_years; }
         // get number of months
     int GetMonths() const { return m_months; }
+        // returns 12*GetYears() + GetMonths()
+    int GetTotalMonths() const { return 12*m_years + m_months; }
         // get number of weeks
     int GetWeeks() const { return m_weeks; }
         // get number of days