Introduce a typedef for frontend handles.

This is another major source of unexplained 'void *' parameters
throughout the code.

In particular, the currently unused testback.c actually gave the wrong
pointer type to its internal store of the frontend handle - it cast
the input void * to a Terminal *, from which it got implicitly cast
back again when calling from_backend, and nobody noticed. Now it uses
the right type internally as well as externally.

terminal.c

@@ -1639,7 +1639,7 @@ const optionalrgb optionalrgb_none = {0, 0, 0, 0};
  * Initialise the terminal.
  */
 Terminal *term_init(Conf *myconf, struct unicode_data *ucsdata,
-		    void *frontend)
+                    Frontend *frontend)
 {
     Terminal *term;