Fixed memory leak of TabIface.

Added documentation of the XS interface on how the Ref<> and Clone<>
wrappers work.

xs/xsp/GUI.xsp

@@ -39,7 +39,7 @@ void add_debug_menu(SV *ui, int event_language_change)
 void create_preset_tabs(bool no_controller, int event_value_change, int event_presets_changed)
     %code%{ Slic3r::GUI::create_preset_tabs(no_controller, event_value_change, event_presets_changed); %};
 
-Ref<TabIface> get_preset_tab(char *name)
+TabIface* get_preset_tab(char *name)
     %code%{ RETVAL=Slic3r::GUI::get_preset_tab_iface(name); %};
 
 bool load_language()

xs/xsp/typemap.xspt

@@ -211,7 +211,6 @@
 %typemap{PresetHints*};
 %typemap{Ref<PresetHints>}{simple};
 %typemap{TabIface*};
-%typemap{Ref<TabIface>}{simple};
 
 %typemap{PrintRegionPtrs*};
 %typemap{PrintObjectPtrs*};