Fix file locking problem on windows (bug 9925)

Fix file locking problem on windows (bug 9925)

External processes cannot access files which are open in LyX. Therefore the
temp files created by the external inset need to be closed right after
creation. The symptom was that the date inset did not produce any outout on
windows (bug 9925). This change reverts a small part of f09a9fe2.
Although the date inset is unimportant and will probably be removed, this
change is important for all external insets that make use of temp files.