applets/mediacontroller: Move PlaceholderMessage to albumRow and load it on demand

1. placeholderMessage and fallbackIconItem are put in the same Loader and
   loaded on demand, and they share the same icon property.
2. When getting "Desktop Icon Name", check if `mpris2Source.currentData` is
   available first.
3. Hide the details column when no track is available.

BUG: 444322
BUG: 446095
FIXED-IN: 5.24