Everything: Add -Wnon-virtual-dtor flag

This flag warns on classes which have `virtual` functions but do not
have a `virtual` destructor.

This patch adds both the flag and missing destructors. The access level
of the destructors was determined by a two rules of thumb:
1. A destructor should have a similar or lower access level to that of a
   constructor.
2. Having a `private` destructor implicitly deletes the default
   constructor, which is probably undesirable for "interface" types
   (classes with only virtual functions and no data).

In short, most of the added destructors are `protected`, unless the
compiler complained about access.

Userland/Applications/SoundPlayer/Player.h

@@ -84,6 +84,8 @@ public:
     PlaybackManager& manager() { return m_player_state.manager; }
 
 protected:
+    virtual ~Player() = default;
+
     PlayerState m_player_state;
     RefPtr<PlaylistModel> m_playlist_model;
 };