Sync the folder after saving, and save from the command line through a temporary file

QSaveFile writes a new file, forces it to disk and renames it over the
old one, but does not force the folder to disk. On ext4 or XFS a power
cut soon after a save can then lose the rename: the previous file comes
back, or none. QET::syncFolderOf() now syncs the folder after every
project, symbol and title block save (QET::writeXmlText()) and every
.qetz save (QetZip::write()). Nothing to do on Windows.

--resave and --set-titleblock wrote a .qet straight over the output file
(QFile, truncate, write): a run stopped part-way left half a project.
They now write through QET::writeXmlText() like the GUI's save. On
Windows their .qet now has LF line ends, as the GUI writes.

Checked with strace on --resave: fdatasync of the new file, linkat into
place, fsync of the folder. The .qet written is byte-identical to before.
ctest 106/106.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmake/qet_compilation_vars.cmake

@@ -941,6 +941,7 @@ set(QET_SRC_FILES
   ${QET_DIR}/sources/utils/qetsettings.h
   ${QET_DIR}/sources/utils/qetutils.cpp
   ${QET_DIR}/sources/utils/qetutils.h
+  ${QET_DIR}/sources/utils/syncfolder.h
 
   ${QET_DIR}/sources/materiallist/materialentrydialog.cpp
   ${QET_DIR}/sources/materiallist/materialentrydialog.h