Save metadata in-place in limited cases

[andy-sweet]

This enables the save button and makes it overwrite the existing metadata file associated with the layer. The motivation is to allow metadata changes that don't require rewriting all the layer data, which could be achieved through the standard Save selected layer ... option in napari.

But it's also a potentially dangerous/lossy operation, so it is only enabled if all the following conditions hold.

1. The layer was read with this plugin. This way we know it's an ome-zarr, we know what file to overwrite (the multiscale image group's zattrs), and can do so with ome-zarr's write_multiscales_metadata function.

2. The layer's file path is writable. I'm assuming this won't typically apply for ome-zarr stored on remote cloud services like S3.

3. The number of axes is the same in the layer and original file. In particular, napari-ome-zarr splits multi-channel images into multiple layers.

In the case that it is not supported, the tooltip of the save button gives a reason why.

Closes #31

[codecov-commenter]

Codecov Report

Merging #36 (e5c1db3) into main (180867b) will decrease coverage by 1.02%.
The diff coverage is 80.00%.

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@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
- Coverage   93.72%   92.71%   -1.02%     
==========================================
  Files          16       16              
  Lines        1371     1469      +98     
==========================================
+ Hits         1285     1362      +77     
- Misses         86      107      +21     

Impacted Files	Coverage Δ
src/napari_metadata/_writer.py	76.54% <68.85%> (-23.46%)	⬇️
src/napari_metadata/_widget.py	94.11% <84.61%> (-0.54%)	⬇️
src/napari_metadata/_tests/test_widget.py	99.65% <100.00%> (+0.04%)	⬆️

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