gfio: update for handling files, connections, states

gfio: update for handling files, connections, states

Big update, changes:

- Notify from client helpers when a client is removed. This helps
  gfio track this.

- Make the connection states more bullet proof. You can now connect
  and disconnect, and expect connect to work again.

- Limit number of files opened through file selection or drag'n drop
  to just 1.

Signed-off-by: Jens Axboe <axboe@kernel.dk>