Fix build with Xcode 10

For some reason, Mac client is the only project that references
libtransmission files as if they were a part of its own project, and
inconsistently so. Make the include/import style uniform an in line with
other projects.

Adjust Mac client include directory settings a bit along the way.

macosx/QuickLookPlugin/GeneratePreviewForURL.m

@@ -1,6 +1,7 @@
 #import <QuickLook/QuickLook.h>
 
-#import "transmission.h"
+#include <libtransmission/transmission.h>
+
 #import "NSStringAdditions.h"
 
 OSStatus GeneratePreviewForURL(void *thisInterface, QLPreviewRequestRef preview, CFURLRef url, CFStringRef contentTypeUTI, CFDictionaryRef options);