Fixed all the broken F# XML comments.
Added XML doc for FileSystemNotesBackend type.
Added using statements to FileSystemNotesBackendTests to ensure timely disposal of objects and not trigger additional events in test cleanup.
Added detection of external file system changes, but not thread safe at present. Also still raises events for own methods.
Correct mistake in test definition.
Ensured Store disposes of backend when it is disposed.
Added using statements to other tests for Store instances.
Fixed data binding on GUI so note summaries should appear again.
Changed reading of file so can handle other processes locking file.
Updated FileSystemNotesBackend to use MailboxProcessor to handle state. This should be thread safe and will facilitate fixing the tests that currently break.