track compute times for each sentence

[siegebell]

additionally:

• indicate long-running commands (e.g. an icon or annotation)
• user-configured alert when a command longer than X minutes completes

[siegebell]

completed:

• each sentence now tracks how long it took to compute
• the UI displays a timer for long running commands in the status bar
• the UI offers an "interrupt" task in the status bar for long-running commands