Add file logging for macOS.

When Gramps is launched from macOS's LaunchServices it doesn't have
a sys.stderr attached so the default stream logger goes to /dev/null.
Use a FileHandler in tht case, writing the log to $TMPDIR/gramps-pid.log.

This will help particularly in analyzing crashes where python shuts down
as there's no crash report in that case.

gramps/grampsapp.py

@@ -127,6 +127,23 @@ if win():
         elif not os.path.isdir(HOME_DIR):
             os.makedirs(HOME_DIR)
         sys.stdout = sys.stderr = open(logfile, "w", encoding='utf-8')
+# macOS sets stderr to /dev/null when running without a terminal,
+# e.g. if Gramps.app is lauched by double-clicking on it in
+# finder. Write to a file instead.
+if mac() and not sys.stdin.isatty():
+    from tempfile import gettempdir
+
+    log_file_name = 'gramps-' + str(os.getpid()) + '.log'
+    log_file_path = os.path.join(gettempdir(), log_file_name)
+    log_file_handler = logging.FileHandler(log_file_path, mode='a',
+                                           encoding='utf-8')
+    log_file_handler.setFormatter(form)
+    log_file_handler.setLevel(logging.DEBUG)
+
+    logger = logging.getLogger()
+    logger.setLevel(logging.WARNING)
+    logger.addHandler(log_file_handler)
+else:
     stderrh = logging.StreamHandler(sys.stderr)
     stderrh.setFormatter(form)
     stderrh.setLevel(logging.DEBUG)