Improve the terminal filter

The new filter comes with these enhancements:

 - Processing is rune-wise rather than byte-wise;
   it assumes UTF-8 input and single-cell wide characters,
   but this condition should be /usually/ satisfied.
 - Unprocessed control characters are escaped, `cat -v` style.
 - A lot of escape sequences is at least recognised, if not processed.
 - Rudimentary preparation for efficient dynamic updates
   of task views, through Javascript.

We make terminal resets and screen clearing commands
flush all output and assume that the terminal has a new origin
for any later positioning commands.
This appears to work well enough with GRUB, at least.

The filter is now exposed through a command line option.

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