Improve support for TABs in wxListBox under MSW

Always set the LB_USETABSTOPS style flag to achieve behaviour more
compatible with other platforms and expand TABs to align them at tab
stops positioned at every 8 characters.

Also add MSW-specific MSWSetTabStops() method allowing to customize tab
stops.

Update the documentation and the sample to demonstrate using TABs.

Closes https://github.com/wxWidgets/wxWidgets/pull/1789

include/wx/msw/listbox.h

@@ -128,6 +128,10 @@ public:
     // extent of all strings is used. In any case calls InvalidateBestSize()
     virtual void SetHorizontalExtent(const wxString& s = wxEmptyString);
 
+    // This is a wrapper for LB_SETTABSTOPS message and takes tab stops in
+    // dialog units, with the same conventions as LB_SETTABSTOPS uses.
+    virtual bool MSWSetTabStops(const wxVector<int>& tabStops);
+
     // Windows callbacks
     bool MSWCommand(WXUINT param, WXWORD id) wxOVERRIDE;
     WXDWORD MSWGetStyle(long style, WXDWORD *exstyle) const wxOVERRIDE;