shell: Use G_DECLARE_*_TYPE macros

Cut down on boilerplate by using the (no longer that) new helper
macros. We don't care about breaking ABI in private libraries, so
use G_DECLARE_FINAL_TYPE even where the class struct used to be
exposed in the header, except for types we inherit from ourselves
(obviously) or where the class exposes any vfuncs (where changes
could affect inheritance in extensions).

src/shell-secure-text-buffer.c

@@ -30,9 +30,6 @@
 
 #include <string.h>
 
-typedef struct _ShellSecureTextBuffer            ShellSecureTextBuffer;
-typedef struct _ShellSecureTextBufferClass       ShellSecureTextBufferClass;
-
 struct _ShellSecureTextBuffer {
   ClutterTextBuffer parent;
   gchar *text;
@@ -41,10 +38,6 @@ struct _ShellSecureTextBuffer {
   guint text_chars;
 };
 
-struct _ShellSecureTextBufferClass {
-  ClutterTextBufferClass parent_class;
-};
-
 /* Initial size of buffer, in bytes */
 #define MIN_SIZE 16