activeTransfersChanged signal is emitted when a transfer is added to
or removed from the activeTransfers list.

This allows to avoid the overhead of fetching the full list of transfers
just to check if any of them is active.

MNotification is no longer maintained.

Remove grouping of transfer notifications, as lipstick will now be
providing the grouping functionality.

[boosting] Use qt5 booster instead of generic one.
[dbus] Systemd file for dbus service so we get dependencies set.
Signed-off-by: Marko Saukko <marko.saukko@jolla.com>