QFileSystemModel: Enable experimental code path for per-file watching

QFileSystemModel had a #ifdefed out experimental code path for watching
single files to track changes in size, which got outdated over time.

Replace the #ifdef 0 by a check for the environment variable
QT_FILESYSTEMMODEL_WATCH_FILES, fix it up and apply some fixes to
related code to make it work:
- Split file names signaled by QFileSystemWatcher on '/' always.
- Do not instantiate QDirIterator on "", which means "current directory"
  and results in mixed-up directories.
- Check on lastModified() in QExtendedInformation::operator==() so
  that changes trigger an update in _q_fileSystemChanged().
- Fix the #ifdefed out part to compile and not to add directories
  to the watcher.

[ChangeLog][QtWidgets][QFileSystemModel] It is now possible to enable
per-file watching by setting the environment variable
QT_FILESYSTEMMODEL_WATCH_FILES, allowing to track for example
changes in file size.

Task-number: QTBUG-46684
Change-Id: Ia5da9170866416c9529251f889814b23d7a7d069
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

0 files changed, 0 insertions, 0 deletions