ShellCorona: ensure screenUiReadyChanged happens for screens without panels

Marco Martinrequested to merge
work/mart/ensureUiReadyChanged into master

right now screenUiReadyChanged was only emitted when all the panels are loaded
this means that screens without panels won't ever emit this, even tough
they are the first one to get ready.
Emit the signal for all screens that are already ready in checkAllDesktopsUiReady()
    
BUG:511131

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