Typeset maths improvements: line breaking, _bar, on list and abs

• Make Generic information for all REDUCE commands fields all optional, resetting to defaults where appropriate. Correct the behaviour to allow the REDUCE Root Directory text field to be empty, and to use the Command Root Directory if it is set.
• Add a Restart REDUCE item to the REDUCE menu to do a full clean restart of the last-run REDUCE command.
• Set a minimum size for the main window.
• Allow space at the bottom of the main display so that the horizontal scroll bar does not obscure the prompt.
• Improve automatic scrolling to the bottom of the display pane.
• Highlight warnings and errors with an appropriate background colour.
• Typeset maths improvements:
  • Improve line breaking.
  • Allow special function symbols to depend on a function's arity and use this for the gamma function to avoid perturbing "excalc.tst".
  • Process _bar in an identifier as an over-bar even if it is followed by digits or _k.
  • Improve display with on list and display matrix assignments as assignments, which now both appear correct for "alg.tst".
  • Display abs(x) as |x|.
  • Remove all superfluous space around commas in algebraic lists and flat printed matrix rows.