qt: fix image response handling
only delete image response on finished
documentation of QQuickImageResponse mentions that a response may only be deleted after QQuickImageResponse::finished signal
quote from QQuickImageResponse::cancel signal
Note: finished() should not be emitted until the response is complete,
regardless of whether or not cancel() was called. If it is called
prematurely, the engine may destroy the response while it is still active
, leading to a crash.
also remove unused classes