wpctl improvements

This MR improves the wpctl tool in several ways:

• Filters will be always shown in the status output, regardless of whether they are smart or not. If they are smart, they will be shown with the smart flag.
• Similar to clear-default, a new clear-persistent command was added to clear persistent settings. If the key is not provided, all persistent settings will be cleared.

Finally, to keep the same behavior as 'default.audio.sink' and 'default.configured.audio.sink', a small change has been done in module-settings. If a persistent setting is removed from the persistent-sm-settings metadata, the change won't be reflected in sm-settings metadata. This is because settings from sm-settings metadata are never meant to be removed (like 'default.audio.sink').