Added show/hide of "Purging volumes" button depending on a state of "wipe_tower"

xs/src/slic3r/GUI/GUI.hpp

@@ -18,6 +18,7 @@ class wxArrayLong;
 class wxColour;
 class wxBoxSizer;
 class wxFlexGridSizer;
+class wxButton;
 
 namespace Slic3r { 
 
@@ -129,6 +130,7 @@ wxString	from_u8(const std::string &str);
 void add_frequently_changed_parameters(wxWindow* parent, wxBoxSizer* sizer, wxFlexGridSizer* preset_sizer);
 
 ConfigOptionsGroup* get_optgroup();
+wxButton*			get_wiping_dialog_button();
 
 }
 }