Typeset maths and REDUCE Configuration Dialogue improvements

• Remove the middle mouse button binding for the pop-up keyboard, since it clashes with the X Window primary paste gesture (although this is not supported).
• Scroll more reliably to the bottom of the REDUCE I/O display window.
• Add a Kill REDUCE item to the bottom of the REDUCE menu. Display an information alert when REDUCE is killed and an error message if this may have failed. Handle failure of REDUCE to start better and check that REDUCE is alive before trying to send it input.
• Improve alerts relating to printing.
• Display the version number on the title bar.
• Add new sections on Editing, Typeset Maths Display and Printing on Linux to the User Guide.
• Typeset maths improvements:
  • Restore compatibility with tmprint so that excalc.tst runs.
  • Use a narrow space to indicate multiplication (except at the end of a line) because with no space x*y is indistinguishable from xy.
  • Output strings as text rather than maths and identifiers using mathit. Support the dfprint switch.
  • Treat trailing digits in an identifier (optionally preceded by _) as a subscript. Treat the final _ in an identifier as introducing a subscript if it is followed by (the name of) a single character. But as a special case, display body_bar as \bar{body} for a single-character body or \overline{body} for a multi-character body.
  • Ensure that operator identifiers are subscripted like non-operator identifiers. Note that line breaking is thoroughly broken!
  • Display repart and impart as \Re and \Im.
  • Display matrices more readably by using \displaystyle for each element and increasing the row spacing to 1.5em.
• REDUCE Configuration Dialogue improvements:
  • Use \ as directory separator in the Windows default configuration.
  • Detect unreadable directories and files in more fields.
  • Substantial revision to fix some subtle misbehaviour.

v2.21 fixes a couple of bugs in v2.2.