Use more reasonable length for wxSlider in wxGTK by default

Use the same 100 DIPs as in wxMSW, to make the default behaviour more
useful.

Also update documentation to explain which size component should, and
should not, be specified when creating the slider.

Closes https://github.com/wxWidgets/wxWidgets/pull/2012

include/wx/gtk/slider.h

@@ -74,6 +74,8 @@ public:
     GtkWidget *m_scale;
 
 protected:
+    virtual wxSize DoGetBestSize() const wxOVERRIDE;
+
     GtkWidget *m_minLabel,*m_maxLabel;
     bool m_blockScrollEvent;