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

interface/wx/datetime.h

@@ -1703,6 +1703,16 @@ public:
     */
     int GetMonths() const;
 
+    /**
+        Returns the combined number of months in this date span, counting both
+        years and months.
+
+        @see GetYears(), GetMonths()
+
+        @since 2.9.5
+    */
+    int GetTotalMonths() const;
+
     /**
         Returns the combined number of days in this date span, counting both
         weeks and days. This doesn't take months or years into account.