message: Fix warning signed/unsigned compare in `IconGrid`
  Using the public `size_type` definiton from the `std::vector` class is
  perhaps the most accurate way to handle the type. The `size_type` is
  typically an alias for `std::size_t`, though could be different for
  custom containers.
diff: @@ -91,6 +91,7 @@ protected:
  private:
  typedef std::vector<IconGridItem> IconItemList;
  + typedef IconItemList::size_type Index;
  private:
  void updateGrid();
  @@ -99,8 +100,8 @@ private:
  void raiseChangedEvent();
  private:
  - int mHighlightIndex = constants::NO_SELECTION; /**< Current highlight index. */
  - int mCurrentSelection = constants::NO_SELECTION; /**< Currently selected item index. */
  + Index mHighlightIndex = constants::NO_SELECTION; /**< Current highlight index. */
  + Index mCurrentSelection = constants::NO_SELECTION; /**< Currently selected item index. */
  int mIconSize = 0; /**< Size of the icons. */
  int mIconMargin = 0; /**< Spacing between icons and edges of the IconGrid. */
  