decoder/Plugin: simplify compile-time initialization

Add a `constexpr` constructor and several `constexpr` methods to
construct a DecoderPlugin at compile time, in a way which allows
adding new methods later without having to edit each plugin.

src/decoder/plugins/DsfDecoderPlugin.cxx

@@ -361,15 +361,7 @@ static const char *const dsf_mime_types[] = {
 	nullptr
 };
 
-const struct DecoderPlugin dsf_decoder_plugin = {
-	"dsf",
-	nullptr,
-	nullptr,
-	dsf_stream_decode,
-	nullptr,
-	nullptr,
-	dsf_scan_stream,
-	nullptr,
-	dsf_suffixes,
-	dsf_mime_types,
-};
+constexpr DecoderPlugin dsf_decoder_plugin =
+	DecoderPlugin("dsf", dsf_stream_decode, dsf_scan_stream)
+	.WithSuffixes(dsf_suffixes)
+	.WithMimeTypes(dsf_mime_types);