Otherwise, there is no way to access it from other threads.

This may be useful for scenarios like napari-imagej, where access to
Python scripting is available from threads other than the blocked one.

This is an internal variable, not intended as public API.

It doesn't need to be "ij_wrapper" or "ij_fixture".
It's OK to be ij!

This test checks that functions registered via
when_imagej_starts get called as part of imagej.init.

This commit adds section 5.2 to the "Convenience methods of
PyImageJ" that describes how to use the when_image_starts
callback mechanism.

This commit introduces a check to determine if the current
running thread is the main thread. If True, then we continue to block
interactive mode on macOS (it is not possible to share the main thread
with GUI event loop needed for the ImageJ GUI). If False, then the
current thread is *not* the main thread (e.g. a jaunched session) and
interactive mode for macOS can proceed.

Use a more descriptive attribute name and value for
when_imagej_starts() test.

This commit adds the pthread library loading exception to
the logger and assumes that the current thread is the main
thread if no pthread library is found.

The default here should be None, not True, otherwise
this test does not actually test for the `_testing`
attribute value.

This commit improves how we clean up the gateway once the UI
is closed in gui mode. Although it is highly unlikely, it is possible
for a user to run into a NameError when deleting the gateway if a user
enters gui mode but is NOT on macos and successfully exits the gui
without crashing the session (see next below, this requires some playing
around). This is resolved by assigning the gateway to None, ensuring
there is *something* to delete if no gateway object is assigned.

This commit also adds an additional check for a running JVM instance
when in gui mode on non-macos systems. This prevents a
`jpype._core.JVMNotRunning: Java Virtual Machine is not running`
error that is produced when the imagej ui visbility is polled but
is no longer available because the JVM has been shut down via
gui interactions.