When a .blend file is dropped into Blender a small menu opens.
In that menu the user can choose between three options: Open, Link and Append.
One "difficulty" in implementing this was the wm.link and wm.append operators for some reason automatically execute when the filepath is set.
Some other similar operators do that as well. I think this behavior should be removed from all those operators.
If someone wants to execute it directly he can just set the right execution context (like EXEC_DEFAULT) so that the file selector is not opened.
There are cases (like this one) when one wants to set the filepath as default but the file selector should still be opened.
Implementation Detail: I'm unsure if it is safe to call the popup_menu function like this. Are the operator props available as long as there is a reference to the operator? I could also make the draw_menu function independent of the operator but it looks nicer if everything is in one class.
