Better handling for FS events in the tree.

This is a fix for #9215. There were edge cases that weren't properly handled in the
FileTreeViewModel and also it appears that the filesystem sends events with just
a directory entry and no added/removed that could indicate that a directory newly
exists.
fix alignment nit
Merge pull request #9235 from adobe/dangoor/9215-ftvm-exception

Better handling for FS events in the tree.