Make more use of convenience methods

FossilOrigin-Name: 89fbd7a1520ecb65809b4c1bd10006ce9bb3a662cb7e7804abaf85d1ee1da63a

src/KeyMapping.m

@@ -1,6 +1,11 @@
 #import "KeyMapping.h"
 
 @implementation KeyMapping
++ (instancetype)mappingWithCode:(int)code name:(OFString *)name
+{
+	return [[self alloc] initWithCode:code name:name];
+}
+
 - (instancetype)initWithCode:(int)code name:(OFString *)name
 {
 	self = [super init];