The event is sent to the client once all initial state is transmitted.
This means the client is able to see the PlasmaWindow completely created
and not in the intermediate state with further updates being pushed after
being created.

The client side API is adjusted to emit the windowCreated signal after
the initial state event is received. In addition if the window is already
unmapped, the signal will never be emitted which means the not valid
windows are not exposed to the client at all.

The tests are adjusted to reflect the new reality, which in most cases
just means removing the comment that this needs to be improved.

There is one kind of unrelated change included: when an empty icon is
set, the client side now creates a QIcon() instead of going through
QIcon::fromTheme. This wrong behavior was exposed now by the auto tests.