KFileItem: add exists() function and use it in KCoreDirLister

Until now processPendingUpdates did not remove files.
It relied on subsequent updates to the parent dir to remove it.
But this is racy, when a file is touched its parent is added to pendingDirectoryUpdates, but if a file is removed as its parent dir is updated, no further updateDirectory will be called, and the file removal get unnoticed.

Since we already did a stat to check for file attribute changes, we can also make sure in processPendingUpdates whether a file was removed and act accordingly.

CCBUG: 244163