Add wxMouseEvent::GetColumnsPerAction().

This is similar to the existing GetLinesPerAction() but is for, surprise,
columns.

Also change the documentation to say that the value returned by both of these
methods is 3 under "most platforms" as some wxOSX currently uses 1 and not 3.

Closes #15239.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74156 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/event.h

@@ -2725,10 +2725,26 @@ public:
 
     /**
         Returns the configured number of lines (or whatever) to be scrolled per
-        wheel action. Defaults to three.
+        wheel action.
+
+        Default value under most platforms is three.
+
+        @see GetColumnsPerAction()
     */
     int GetLinesPerAction() const;
 
+    /**
+        Returns the configured number of columns (or whatever) to be scrolled per
+        wheel action.
+
+        Default value under most platforms is three.
+
+        @see GetLinesPerAction()
+
+        @since 2.9.5
+    */
+    int GetColumnsPerAction() const;
+
     /**
         Returns the logical mouse position in pixels (i.e.\ translated according to the
         translation set for the DC, which usually indicates that the window has been