macOS: move deleted file to Trash

As opposed to permanently deleting it.  As requested:

  https://sourceforge.net/p/tuxpaint/feature-requests/148/

src/ios.m

@@ -20,6 +20,7 @@
   (See COPYING.txt)
 */
 #include <string.h>
+#include <unistd.h>
 #include <sys/stat.h>
 #include <libgen.h>
 #include <limits.h>
@@ -86,3 +87,9 @@ const char *apple_picturesPath(void)
 {
     return IOS_PICTURES_PATH;
 }
+
+
+int apple_trash(const char *path)
+{
+    return unlink(path);
+}