Undo/Redo commands

Finally got around to implementing undo/redo for commands, which have been sitting there as a TODO since the GUI was first written. Undo/Redo is supported for:

• Merge scenes
• Merge batches
• Split scene
• Split / Auto-split batch
• Merge lines
• Delete lines (a new command, the motivation to finally implement undo)
• Reparse translation (also new)

Undo/Redo is NOT supported for the following commands, and they will clear the undo stack.

• Load project / subtitles
• Translate selection
• Start/resume translation

Undo is also not supported for these operations, as they are not technically commands. The next release will probably convert them to undoable commands.

• Edit scene/batch description
• Edit subtitle text/translation

You can hover over the undo/redo icons in the toolbar to see what type of command will be undone or redone, if in doubt.

There were a lot of technical changes to support undo/redo, so as with any major update there are bound to be new bugs. The automated test suite has been greatly expanded though and quite a few bugs were squashed, so hopefully it is pretty stable. As ever, please report an issue if something isn't working as you would expect it to.

This release also adds a pre-processor option to merge lines shorter than a given length with the previous line. This is to help with the add absurdly short lines (<0.2 seconds) Whisper generates on occasion, but it's not really possible to know whether those lines are an orphaned part of the previous line or just a random hallucination, so this is definitely not guaranteed to improve the result, so it is not enabled by default (the duration is set to 0 seconds).

What's Changed

• Processing settings by @machinewrapped in #179
• Delete line action, plus undo implemented for various commands by @machinewrapped in #180

Full Changelog: v0.7.10...v0.8.0