[4/5] ole32/ole2: After IDropTarget::DragEnter() failure target doesn't count as valid

[Nikolay Sivov]

After IDropTarget::DragEnter() failure target doesn't count as valid

I have a test for this and next patch. It uses a separate thread to produce
mouse messages and works fine on XP, but I wasn't able to run it with Wine due
capturing related problems I think.

Anyway observed behaviour is: 

- if DragEnter fails no target methods are called after that;
- if it succeeded Drop or DragLeave is choosed depending on effect set in it (next patch is about).

P.S. I'll provide test diff of course if somebody is interested.