[Lazarus] Generalized DragDrop

[Hans-Peter Diettrich]

Michael Van Canneyt schrieb:

> I know about the File DND, but as far as I know, it's only on form 
> level, it could be extended to any control (TFileNameEdit jumps to mind).

Windows platform requires a HWND for a file DND, that can be supplied by 
any TWinControl. Other platforms can have different requirements, and we 
should implement only what's possible on all platforms.

When a DragObject is used in such dragging, it contains the concrete 
target control at the time of the drop. Then it's only a matter of a LCL 
convention, which controls qualify as dropfiles targets, e.g. when they 
have an OnDropFiles handler installed.

DoDi