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

src/gtk1/window.cpp

@@ -1332,6 +1332,7 @@ template<typename T> void InitMouseEvent(wxWindowGTK *win,
     if (event.GetEventType() == wxEVT_MOUSEWHEEL)
     {
        event.m_linesPerAction = 3;
+       event.m_columnsPerAction = 3;
        event.m_wheelDelta = 120;
        if (((GdkEventButton*)gdk_event)->button == 4)
            event.m_wheelRotation = 120;