Port macOS client to C++ (#1830)

* Rename all .m (Obj-C) files to .mm (Obj-C++)

* Fix build in Obj-C++ mode

* Fix Xcode build

macosx/ExpandedPathToPathTransformer.mm

@@ -0,0 +1,42 @@
+/******************************************************************************
+ * Copyright (c) 2007-2012 Transmission authors and contributors
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ *****************************************************************************/
+
+#import "ExpandedPathToPathTransformer.h"
+
+@implementation ExpandedPathToPathTransformer
+
++ (Class)transformedValueClass
+{
+    return [NSString class];
+}
+
++ (BOOL)allowsReverseTransformation
+{
+    return NO;
+}
+
+- (id)transformedValue:(id)value
+{
+    return value == nil ? nil : [NSFileManager.defaultManager displayNameAtPath:value];
+}
+
+@end