Add KWindowSystem::updateStartupId(QWindow *window)
This util method should allow applications to not have to repeat all the platform specific code, including the need of additionally linking to Qt5::X11Extras with all the extra build system setup.
See e.g. how this would cut away most of things done in network/konversation!69 (diffs)
Sadly kwayland-integration is released in another cycle with Plasma, so what to do moving the platform-implementation instead into the respective plugins?
The previous idea of having some KWindowSystem::activateWindowAsIfLaunched() is discarded for now from my side, as applicaations might have own ideas what to do with any existing windows of them, if "launched" another time. But all would need to do what this proposed util method KWindowSystem::updateStartupId() does.