Improve device unmount/eject messages

While doing some udisks development and testing with Thunar, I've noticed Thunar doesn't bring up very helpful device removal messages.

Currently, when the eject icon on a device is clicked, Thunar brings up a notification (thunar/thunar-notify.c function thunar_notify_unmount()) with:

Writing data to device
There is data that needs to be written to the "
"device \"%s\" before it can be removed. Please "
"do not remove the media or disconnect the drive"

which has an infinite timeout. Then, when the unmount is completed, thunar_notify_finish() reduces the timeout to 2 more seconds. Then it just disappears. The user is never told when it is safe to remove the device.

glib's gio's GMountOperation has the "show-unmount-progress" signal since version 2.34 (released 24 September 2012). These tell us what messages to show the user, and at the end we get one with:

[Device] can be safely unplugged
Device can be removed

It delivers this with i18n already applied. We should probably use it.