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

include/wx/event.h

@@ -1747,9 +1747,13 @@ public:
     wxMouseWheelAxis GetWheelAxis() const { return m_wheelAxis; }
 
     // Returns the configured number of lines (or whatever) to be scrolled per
-    // wheel action.  Defaults to one.
+    // wheel action. Defaults to three.
     int GetLinesPerAction() const { return m_linesPerAction; }
 
+    // Returns the configured number of columns (or whatever) to be scrolled per
+    // wheel action. Defaults to three.
+    int GetColumnsPerAction() const { return m_columnsPerAction; }
+
     // Is the system set to do page scrolling?
     bool IsPageScroll() const { return ((unsigned int)m_linesPerAction == UINT_MAX); }
 
@@ -1770,6 +1774,7 @@ public:
     int           m_wheelRotation;
     int           m_wheelDelta;
     int           m_linesPerAction;
+    int           m_columnsPerAction;
 
 protected:
     void Assign(const wxMouseEvent& evt);