Move actual password derivation to separate class

LegacyPasswordGenerator.h

@@ -0,0 +1,18 @@
+#import <ObjFW/ObjFW.h>
+
+@interface LegacyPasswordGenerator: OFObject
+{
+	size_t _length;
+	OFString *_site;
+	const char *_passphrase;
+	unsigned char *_output;
+}
+
+@property size_t length;
+@property (copy) OFString *site;
+@property const char *passphrase;
+@property (readonly) unsigned char *output;
+
++ (instancetype)generator;
+- (void)derivePassword;
+@end