This patch makes the kded daemon monitor also newly connected outputs,
and leads to correctly initialize outputs from a docking station plugged
in after session start.

For "internal" display connector ports (HDMI, DisplayPort, etc.
integrated in the system, we know from the start that they exist. These
are marked disconnected. When plugging in a docking station, new outputs
(that aren't previously known as disconnected) appear. daemon.cpp only
monitors outputs that change connection state, so it doesn't consider
newly added outputs.

By listening to KScreen::Config::outputAdded, we can detect new output
(connectors) as well, so we can trigger the applyConfig code path, which
leads to restoring the config.

BUG:366346

• Reproduced the issue in https://bugs.kde.org/show_bug.cgi?id=366346 and

confirmed it fixes the problem on my system (Lenovo Onelink+ docking
station which shows the same behaviour).

• Added debug calls and confirmed the new outputs are correctly set up.

auto-testing this seems to be close to impossible. :/