Add transcript editor

[chidiwilliams]

• Add editing to transcript viewer
• Undo and redo when editing transcript

Resolves #404

[codecov]

Codecov Report

Merging #410 (d64bc05) into main (b4a095e) will increase coverage by 0.57%.
The diff coverage is 98.89%.

@@            Coverage Diff             @@
##             main     #410      +/-   ##
==========================================
+ Coverage   78.14%   78.71%   +0.57%     
==========================================
  Files          15       21       +6     
  Lines        1917     2030     +113     
==========================================
+ Hits         1498     1598     +100     
- Misses        419      432      +13     

Flag	Coverage Δ
Linux	78.47% <98.34%> (+0.59%)	⬆️
Windows	75.47% <98.34%> (+0.70%)	⬆️
macOS	77.04% <98.89%> (-0.22%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
buzz/action.py	92.85% <92.85%> (ø)
buzz/widgets/transcription_viewer_widget.py	98.68% <98.68%> (ø)
buzz/gui.py	76.71% <100.00%> (-1.63%)	⬇️
buzz/icon.py	100.00% <100.00%> (ø)
buzz/paths.py	100.00% <100.00%> (ø)
buzz/widgets/toolbar.py	100.00% <100.00%> (ø)
...zz/widgets/transcription_segments_editor_widget.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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