Fixed several warnings

The biggest difference is changing PresetCollection::m_idx_selected from int to size_t, which fixed many warnings at once.

src/slic3r/GUI/GUI_ObjectList.cpp

@@ -479,7 +479,6 @@ void ObjectList::update_extruder_values_for_items(const size_t max_extruder)
 
 void ObjectList::update_objects_list_extruder_column(size_t extruders_count)
 {
-    if (!this) return; // #ys_FIXME
     if (printer_technology() == ptSLA)
         extruders_count = 1;