Add native icon support

[ygoe]

To show an icon with each tree item, the user needs to create a template that includes that additional Image element. This is normally included in the item's selection border and thus doesn't look good in Classic-like themes (Luna etc.). Instead, and to make the use of icons easier, the icon image could be added to the default item template. Aero and Classic themes could then distinguish as to whether or not include the icon in the selection border.

While WPF offers the flexibility to include virtually anything with tree items (like 2 checkboxes and 3 images), the most often found use case is arguably text with a single icon image. This main scenario could be easier. If the additional flexibility is needed, the icon doesn't need to be used and a custom template can be defined at any time.