Revert "Use QTemporaryFile instead of some homebrew construction."

This reverts commit 23d0dc225dbe3f89d0411c0195d8bcd066c3bdc7.
Thought the implementation looks right in the first place, it
will fail if we are asking for a random name inside a directory
where we do not have access rights. QTemporaryFile will not
roundtrip to the admin server and therefor return an empty string.
Change-Id: I0bd716b10e43242f5da5dd939b06213040256d67
Reviewed-by: Niels Weber <niels.weber@digia.com>