QMimeData::retrieveData() documentation needs improvement

We need to know the moment of drop to the filesystem. This is necessary for time-consuming file creation operation and has to be performed only once.

Signal "dataRequested" is sent lots of times, when drag object leaves the bound of the app and data requested multiple time. We need to know the moment of actual drop to start creating big file with the data and only once, per drag/drop operation.

At the moment, the data is requested more than once and we just can't start creating files, since we do not know when to do this.