feat(file-compare): compare files in the background with progress and a log console - #67

Summary

Comparing large files used to freeze the window until the comparison finished. Now the comparison runs in the background: the window stays responsive and a progress bar shows how far it has got.

File comparison windows now also have a log console, a pane at the bottom of the window. It shows read errors, which no longer appear in a blocking alert, and how long each comparison took.

What changes for the user

• Background comparison with progress
  • the status bar shows which file is being read, then a progress bar while the comparison runs
  • the current comparison stays visible until the new one is ready
  • toolbar buttons, menus and path controls are disabled while the comparison runs, then re-enabled
  • a request made during a comparison (for example a path change or a file drop) is queued and runs right after the current one
• Log console
  • each completed comparison is logged with the time it took
  • read errors are logged too, the console opens automatically, and the previous comparison stays on screen
  • it can be shown or hidden from View ▸ Show Log Console (⌥⌘L)
• Unsaved edits are safer: before anything reloads the files (drag & drop, a path change, session preferences, a file changed outside the app), you're asked to save your edits. Cancelling keeps them.
• Navigation: moving to the next or previous file of a folder session no longer marks the session as modified.
• Faster on large files: very long or minified lines are handled faster, and resizing the window no longer recalculates row heights unless it needs to.