No longer return fixed values from wxGauge::GetBestSize() in wxGTK

Under wxGTK, wxGauge was returning fixed values for height and width.
This meant that the gauge would not center correctly in a sizer,
particularly under GTK+ 3 where the default gauge height is just a few
pixels.

Following this change, wxGauge renders correctly on GTK+ 3 and matches
the reference widget display in the gtk3-widget-factory app.

Note, this change will also result in a slimmer widget on GTK+ 2, but
the gauge height can be forced using wxGauge::SetMinSize() on the older
toolkit.

The gauge presentation is totally theme dependent under GTK+ 3. For
example, the user can have thicker gauges by setting the following in
their ~/.config/gtk-3.0/gtk.css file:

progress, trough {
  min-height: 20px;
}

Closes https://github.com/wxWidgets/wxWidgets/pull/1353

include/wx/gtk/gauge.h

@@ -65,8 +65,6 @@ protected:
     // set the gauge value to the value of m_gaugePos
     void DoSetGauge();
 
-    virtual wxSize DoGetBestSize() const wxOVERRIDE;
-
 private:
     void Init() { m_rangeMax = m_gaugePos = 0; }