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/PixelPaint/Image.h

@@ -49,6 +49,9 @@ public:
     virtual void image_did_modify_layer_stack() { }
     virtual void image_did_change() { }
     virtual void image_select_layer(Layer*) { }
+
+protected:
+    virtual ~ImageClient() = default;
 };
 
 class Image : public RefCounted<Image> {

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;
 };

Userland/Applications/SoundPlayer/VisualizationBase.h

@@ -32,4 +32,7 @@ class Visualization {
 public:
     virtual void set_buffer(RefPtr<Audio::Buffer> buffer) = 0;
     virtual void set_samplerate(int) { }
+
+protected:
+    virtual ~Visualization() = default;
 };

Userland/Applications/SpaceAnalyzer/TreeMapWidget.h

@@ -37,6 +37,9 @@ struct TreeMapNode {
     virtual size_t num_children() const = 0;
     virtual const TreeMapNode& child_at(size_t i) const = 0;
     virtual void sort_children_by_area() const = 0;
+
+protected:
+    virtual ~TreeMapNode() = default;
 };
 
 struct TreeMap : public RefCounted<TreeMap> {