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/msw/missing.h

@@ -86,6 +86,9 @@
     #ifndef SPI_GETWHEELSCROLLLINES
         #define SPI_GETWHEELSCROLLLINES 104
     #endif
+    #ifndef SPI_GETWHEELSCROLLCHARS
+        #define SPI_GETWHEELSCROLLCHARS 108
+    #endif
 #endif // wxUSE_MOUSEWHEEL
 
 // Needed by window.cpp