Download/Upload notification: show only one and update this

[tobiasKaminsky]

After starting multiple up/downloads show only one notification and update this:
(1 / 50) $current file: 20kb / 10Mb

[rytilahti]

👍 this is quite annoying especially when combined with kdeconnect for desktop notifications.
edit: #725 is related.

[tobiasKaminsky]

To have this a bit easier, let us use one notification for upload and one for download.
This way we have at max 2 changing notifications, but is way lot easier to update the info in it.