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/x11/window.cpp

@@ -1486,6 +1486,7 @@ bool wxTranslateMouseEvent(wxMouseEvent& wxevent,
                     button = xevent->xbutton.button;
 
                     wxevent.m_linesPerAction = 3;
+                    wxevent.m_columnsPerAction = 3;
                     wxevent.m_wheelDelta = WHEEL_DELTA;
 
                     // Button 4 means mousewheel up, 5 means down