We would first invoke the dragging, and then set the drag data (like the
ID or the dragged modifier), so the `wmDropBox.on_drag_start()` handler
wouldn't be able to access this. This broke dragging some IDs from the
Outliner, noticed in D15333.
It's now possible to first create/request drag data, extend it, and then
invoke the actual dragging. The normal function to start dragging
returns `void` now instead of `wmDrag *`, so the drag data can't easily
be modified after starting anymore.
Moves code for managing dragging data from buttons to a separate file.
This way all this closely related code is in one location, making it
easier to see how it all relates, and easier to find.
