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/gtk/window.cpp

@@ -1649,6 +1649,7 @@ window_scroll_event(GtkWidget*, GdkEventScroll* gdk_event, wxWindow* win)
 
     // FIXME: Get these values from GTK or GDK
     event.m_linesPerAction = 3;
+    event.m_columnsPerAction = 3;
     event.m_wheelDelta = 120;
 
     // Determine the scroll direction.