Always initialize the gateway attribute

The global gateway attribute is only created on macOS
in GUI mode. This commit moves the gateway attribute to
the module scope and attaches the gateway from the
_create_gateway() for all modes. This makes it possible
to access the last initialized ImageJ gateway by accessing
the gateway attribute of the imagej module.

src/imagej/__init__.py

@@ -60,6 +60,7 @@
 __author__ = "ImageJ2 developers"
 __version__ = sj.get_version("pyimagej")
 
+gateway = None
 _logger = logging.getLogger(__name__)
 _init_callbacks = []
 _rai_lock = threading.Lock()
@@ -1224,12 +1225,10 @@ def run_callbacks(ij):
             callback(ij)
         return ij
 
+    global gateway
     if mode == Mode.GUI:
-        # Show the GUI and block.
-        global gateway
 
         def show_gui_and_run_callbacks():
-            global gateway
             gateway = _create_gateway()
             gateway.ui().showUI()
             run_callbacks(gateway)
@@ -1267,7 +1266,9 @@ def show_gui_and_run_callbacks():
         return gateway
 
     # HEADLESS or INTERACTIVE mode: create the gateway and return it.
-    return run_callbacks(_create_gateway())
+    gateway = _create_gateway()
+
+    return run_callbacks(gateway)
 
 
 def when_imagej_starts(f) -> None: