Re-add method to get a temporary file name without persistent QTemporaryFile object

Re-add method to get a temporary file name without persistent QTemporaryFile object

This is needed for cases where the temp file has to be manually removed
at some point (e.g., if temp files are used as conversion target, and
the initial file only serves as a placeholder), since QTemporaryFile
objects cannot be manually removed at least on Windows (they are always
kept open internally even after close()). See
​http://lists.qt-project.org/pipermail/interest/2013-August/008352.html

In order to avoid race conditions due to duplicate names (the issue why
the old method was removed), we record all used temp file names.

Fixes: #9139