Progress bar issues

[DikieDick]

• Progress bar should remain filled after checking is completed.
• Progress bar could indicate more precisely what coq-lsp is currently thinking about.
• Improve relative positioning of the progress bar endpoint. Sometimes, the progress bar seems quite far off, see for instance the sheet sheet4_excluded_middle.mv, where coq-lsp is thinking quite a bit about the last contradiction, but the progress bar stops even before the exercise.