fix issue #85
Merge pull request #86 from punker76/85-drop-to-textbox

fix issue #85
fix issue #84

- it's now possible to select combox and items, and use the srollbar and
mousewheel
- introduce CanStartDrag method for IDragSource to handle starting
drag&drop
Merge pull request #87 from punker76/84-listview-item-with-combobox

fix for issue #84
VisualTreeHelper.HitTest allows only Visual
Merge pull request #94 from punker76/92-wrong-cast-for-HitTest

fix #92 Unable to cast object of type 'System.Windows.Documents.Run' to type 'System.Windows.Media.Visual'