On macOS with headless=False, show the GUI

This uses jpype.setupGuiEnvironment. It works, but:

* You must have pyobjc installed from pip
  (conda packages did not work for me).

* With this change, it is now inconsistent whether
  imagej.init(headless=False) pops a GUI or not. On macOS yes, on
  Linux and Windows no, with ij.ui().showUI() needing to be called.

* This change does not fix the fact that imagej.init(headless=False)
  is non-blocking on Linux and Windows, but blocking on macOS. This
  is because on macOS the main thread must eternal stay busy doing
  the console event loop from this point forward.

See #23.

imagej/__init__.py

@@ -42,7 +42,7 @@
 
 from pathlib import Path
 
-from jpype import JArray, JException, JImplementationFor, JObject
+from jpype import JArray, JException, JImplementationFor, JObject, setupGuiEnvironment
 
 from .config import __author__, __version__
 
@@ -121,7 +121,11 @@ def init(ij_dir_or_version_or_endpoint=None, headless=True, add_legacy=True):
     ij = imagej.init('sc.fiji:fiji', headless=False)
     """
     _create_jvm(ij_dir_or_version_or_endpoint, headless, add_legacy)
-    return _create_gateway()
+    if sys.platform == 'darwin' and not headless:
+        # NB: This will block the calling (main) thread forever!
+        setupGuiEnvironment(lambda: _create_gateway().ui().showUI())
+    else:
+        return _create_gateway()
 
 
 def _create_jvm(ij_dir_or_version_or_endpoint, headless, add_legacy):