Provide better access to local ImageJ2 installation

[gselzer]

This PR provides the ability for users to:

1. Specify a local ImageJ2 installation instead of loading a stock ImageJ2 (which, if necessary, will be downloaded at the latest version)
2. If possible (i.e. if PyImageJ can run interactively) show the ImageJ2 GUI.

Limitations:

1. Spawning the ImageJ2 GUI and then proceeding to close that ImageJ2 GUI closes napari. More investigation needed into why it is happening. Ideally, ImageJ2's close button would just hide the ImageJ2 GUI, such that you can still make napari-imagej calls after you close the ImageJ2 GUI (and potentially reopen the GUI at a later point).
2. It is difficult to share images between the ImageJ2 "head-ful" behavior and napari. For example, if you use a local Fiji installation, you will see the classic "Blobs" command, and you can run it to get the "Blobs" image. But that image is displayed in a new layer. I'd want to see it displayed in napari instead. I'm assuming we can accomplish this with some new Service implementation (right @ctrueden?), but I don't know which yet.
3. There is no guarantee that any third-party plugins will actually work. For example, using my local Fiji installation will provide functionality like BigDataViewer to napari-imagej. Running BigDataViewer presents the startup screen, but pressing Browse to search for an XML file will freeze both the popup and napari more generally, requiring me to kill napari. Is there a way that we can disclaim the limitations of third-party plugins if we can't support all of them?

TODO:

• Test that we can specify a local Fiji installation in pytest
• Ensure that "interactive" mode is used on Windows and Linux, but not on Mac
• Make it clear why the ImageJ2 GUI button is disabled only on Mac.

Closes both #16 and #43

[codecov-commenter]

Codecov Report

Merging #89 (40f3620) into main (14ff815) will decrease coverage by 0.87%.
The diff coverage is 85.02%.

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
- Coverage   93.36%   92.48%   -0.88%     
==========================================
  Files          15       17       +2     
  Lines        2501     2849     +348     
==========================================
+ Hits         2335     2635     +300     
- Misses        166      214      +48     

Impacted Files	Coverage Δ
src/napari_imagej/_module_utils.py	90.35% <0.00%> (+0.64%)	⬆️
src/napari_imagej/widget_IJ2.py	83.94% <83.94%> (ø)
tests/test_IJ2GUIWidget.py	84.43% <84.43%> (ø)
src/napari_imagej/setup_imagej.py	95.74% <90.47%> (-0.44%)	⬇️
src/napari_imagej/widget.py	84.72% <100.00%> (+0.21%)	⬆️
tests/conftest.py	100.00% <100.00%> (ø)
tests/test_ImageJWidget.py	98.67% <100.00%> (+0.01%)	⬆️

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[gselzer]

TODO before merge:

• Add documentation for what happens when ImageJ2 GUI is closed
• Add issue for fixing ImageJ2 GUI post close disabling napari-imagej.
• Move settings.yml to a user-space config directory. Ideally we could lean on a library for this. Make an issue for this and solve it there.
• Also mention the scripts folder. That should be a setting too.
• Remove the ImageJ UI widget, and squish it into the main widget.