#271 problem with drag and drop of files in project tree view

[Marek Januszewski]

When the project tree is larger than the project
window, if I try to drag the item in the list over the
very first visible item in the tree view, the window
doesn't scroll up making it impossible to drop the item
anywhere above first visible tree node in the project
tree. Once the object is dragged over the very top
visible item, the project tree view should scroll up.