Allow QFile.copy() to already existing file (make it compatible with QTemporaryFile)

The simple idea to copy a file to a temporary file will not work with the following code:

QFile* source_file;
...
QTemporaryFile* tmp_file = new QTemporaryFile();
tmp_file->open(); tmp_file->close(); //make name

source_file->copy(tmp_file->fileName());

Because tmp_file->open() generates an empty file, which lets QFile.copy() fail.

Solution: allow copy() to already existing files (overwrite mode).