applets/devicenotifier: Make "removing" message more instructional
When you unmount a device using the UI, the device shows a message saying, "Removing..." This message remains visible as long as the device is still processing file move or copy operations due to the use of async file IO. The device is not yet safe to unplug as long as the message is visible, but this is not obvious from the UI. As a result, it is possible for people to think it's stuck and unplug it anyway, causing data loss.
This commit attempts to address that by showing a different message when the unmount takes more than one second, which tells the user what is actually happening and explicitly asks them not to unplug it yet.
For unmounts that take under one second, there is no change.
