Fix race condition when saving qmlproject file

Sometimes the project stops updating after making changes to .qmlproject project file.
This is caused by a race condition in file system watcher. When a file is saved, we use "safe save" mechanism - the file contents is written to a temporary which the replaces the original file.
The problem is this replacement is not atomic and the file system watcher detects that the tracked file is removed. When this happens, the file system watcher stops tracking the file.
We try to mitigate this by attempting to re-add the file if it still exists, but sometimes (especially in smaller projects, where there are not so many tracked files) this check is made to soon and then we stop tracking the file definitively.