Fixed several warnings

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

src/libslic3r/GCode/ToolOrdering.hpp

@@ -72,7 +72,7 @@ private:
     std::map<const ExtrusionEntity*, ExtruderPerCopy> entity_map;  // to keep track of who prints what
     bool something_overridable = false;
     bool something_overridden = false;
-    const LayerTools* m_layer_tools;    // so we know which LayerTools object this belongs to
+    const LayerTools* m_layer_tools = nullptr;    // so we know which LayerTools object this belongs to
 };