[OSD] Fix ProgressBar potentially growing the dialog

Depending on font size and settings the ProgressBar's implicit size might be wider
than our minimum size of 15 gridUnit. This causes the Popup to change size when switching
from ProgressBar (e.g. changing volume) to Text (e.g. volume muted).

By setting preferredWidth to 1 we ensure we never exceed the overall minimum size
when the ProgressBar is shown.