Every job is now defined in a dedicated table with a "command" key
defining the command to be run. Other keys and tables without a
command key are ignored. All uppercase keys in the root table can be
used to define variables. Placeholders in the form $KEY or ${KEY} in
the command strings are substituted with the respective values.

Implement loggig to file.  Add status indicator widget next to
terminal widgets reporting the status of the command being executed.
Improve application layout.

Equal to 0.2 of the line height and shift canvas rendering upward by
the same amount. Because of how line height is calculated, this makes
the text appear better centered in the widget.