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.

[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>

Previously, the transfer engine wasn't invoked, but instead launched
directly.  This means that it would not have privileges.