wayland: Fix output hotplugging

When an output is hotplugged, kscreen kded module will find a matching config and check whether it can be applied by calling Config::canBeApplied().

The Config::canBeApplied() function will check whether the current output setup satisfies various properties specified in the config, e.g. modes, etc. One of those things is the maximum number of active outputs - Screen::maxActiveOutputsCount().

When an output is added, the backend needs to call Screen::setMaxActiveOutputCount() to ensure that the kded module will be able to properly determine whether a config candidate is applicable.

With output device v2 changes, one of the setters got lost, this change brings it back. https://invent.kde.org/plasma/libkscreen/-/blob/Plasma/5.22/backends/kwayland/waylandconfig.cpp#L153