microphoneindicator: keep SourceModel->isMuted() in sync with the model

Nate Grahamrequested to merge
ngraham/plasma-pa:synced-mutes into master

We query this property to determind if the default input is muted, so we need to set it appropriately when we update our model.

BUG: 472107 FIXED-IN: 6.0

@broulik

Edited by Nate Graham

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