In theory the dmon based directory monitoring is enough to ensure that
the list of project files is always correct. In reality some events
may be missing and the project files list may get disaligned with the
real list of files.
To avoid the problem we add an additional rescan to be done later in a
thread on any project subdirectory affected by an event of directory of
file change.
In the rescan found the same files already present the thread terminates.
If a difference is found the files list is modified and a new rescan is
scheduled.
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