Add un-/re-do and save-/load-states for proofreading tool

[qin-yu]

This PR add mechanisms to store snapshots during proofreading, fixes #244

Screenshot: New interface

• "Undo" function and button
• "Redo" function and button
• Save progress to disk and load
• Re-initialise with different segmentation/state file
• Use different boundary images as reference for split/merge
• Group buttons into a compact unit

Details

• 10 snapshots are tracked by default, set PLANTSEG_MAX_UNDO_ACTIONS=30 to allow extra snapshots or PLANTSEG_MAX_UNDO_ACTIONS=3 to decrease memory usage.
• Now "Re-initialise" will force reset and re-initialisation after a warning
• Now the reference boundary image can be changed at each split/merge

Screenshot: Users are warned before destructive processes

[codecov]

Codecov Report

Attention: Patch coverage is 33.08824% with 91 lines in your changes missing coverage. Please review.

Project coverage is 65.62%. Comparing base (022e03d) to head (8ac7db1).
Report is 12 commits behind head on plantsegv2.

Files with missing lines	Patch %	Lines
plantseg/viewer_napari/widgets/proofreading.py	32.08%	91 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           plantsegv2     #323      +/-   ##
==============================================
- Coverage       66.27%   65.62%   -0.65%     
==============================================
  Files              67       67              
  Lines            4587     4675      +88     
==============================================
+ Hits             3040     3068      +28     
- Misses           1547     1607      +60     

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