Add check if running on main thread for macOS

Jaunch now enables a true interactive experince
for macOS users, but only if PyImageJ is launch in
a seperate thread (i.e. not the main thread). This commit
adds an additional check for which thread we are currently on.

If on the main thread, fail like before and prevent interactive
mode on macOS. Otherwise, proceed as usual.

src/imagej/__init__.py

@@ -1205,7 +1205,7 @@ def init(
 
     macos = sys.platform == "darwin"
 
-    if macos and mode == Mode.INTERACTIVE:
+    if macos and mode == Mode.INTERACTIVE and _is_main_thread():
         raise EnvironmentError("Sorry, the interactive mode is not available on macOS.")
 
     if not sj.jvm_started():
@@ -1517,6 +1517,14 @@ def _includes_imagej_legacy(items: list):
     return any(item.startswith("net.imagej:imagej-legacy") for item in items)
 
 
+def _is_main_thread():
+    """Detect if on main thread running on the main thread.
+
+    :return: Boolean indicating if the current thread is the main thread.
+    """
+    return threading.current_thread() == threading.main_thread()
+
+
 def _set_ij_env(ij_dir):
     """
     Create a list of required jars and add to the java classpath.