Code cleanup: remove useless wx/wx.h include inside a few files.

in most of files, including wx.h is not necessary, when only 2 or 3 wx files must be included.
Moreover, on windows, including wx.h sometimes create compil warnings about
shadowed vars defined in some specific windows headers.

common/widgets/mathplot.cpp

@@ -631,7 +631,7 @@ void mpFXY::Plot( wxDC& dc, mpWindow& w )
         if( !m_continuous )
         {
             bool first = true;
-            wxCoord ix;
+            wxCoord ix = 0;
             std::set<wxCoord> ys;
 
             while( GetNextXY( x, y ) )