Fix T42760: Drag-to-add feature fails in a case.

We have to unset 'name' when we set 'filepath', and vice-versa.
More fixes related to D&D:

* Same issue as T42760 was also reproducible in some cases in 3DView.
* You got an error message about missing RNA prop in some drop-called operators.
* You could not drop a movie file in nodes, (some cases of) 3DView, nor ImageEditor.
Cleanup: warnings & space
Fixed T35128. Progress bar sync on Windows.

Apply for all windows instead of active, otherwise progress doesn't update.
Merge remote-tracking branch 'upstream/master'