gvfsjobunmount: always send unmount notification

Currently unmount notifications ("show-unmount-progress") are only sent if the unmount is asynchronous and lasts longer than 1500 ms.

Send unmount notification even for shorter, synchronous unmounts.

(I am not sure whether this is a good idea... The udisks2 daemon always sends unmount notifications. For some devices, like cameras, it might be a good idea to always notify, since they need to be manually unplugged. When unmounting sftp in Nemo, the notification just flickers on and off, since the reply is quick, but maybe it would be better if the notification stayed for a while and changed to " unmounted".)