Fixed resetting multiselection on drag operation.

Do not reset selection on left mouse down, it breaks drag and drop. Instead do it on mouse up, if no drag operation happened.
Fixed unloading of BorderSelectionLogic

SelectionStrategy was loaded in OnApplyTemplate and unloaded on OnUnloaded event, but sometimes OnUnloaded happens without further OnApplyTemplate. Instead dispose selection strategy in OnApplyTemplate before applying new one.