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/Libraries/LibTextCodec/Decoder.h

@@ -33,6 +33,9 @@ namespace TextCodec {
 class Decoder {
 public:
     virtual String to_utf8(const StringView&) = 0;
+
+protected:
+    virtual ~Decoder() = default;
 };
 
 class UTF8Decoder final : public Decoder {