Changes to 0.7.7

* Add IsPositiveDefinite, IsPositiveSemidefinite, QuarticFormula
* SEMANTICS: QuadraticFormula, CubicFormula, Eigenvalues, and DiagonalOf
  now consistently return a column vector
* Fix CubicFormula for complex coefficients and better handle real roots,
  and use QuarticFormula in PolynomialRoots
* Eigenvalues now computes eigenvalues for 3x3 and 4x4 matrices
* Add undo/redo capability
* CHANGE: It's RayleighQuotient not RaleighQuotient
* "Show Full Answer" menu item to show the full answer in a
  scrollable text window
* "Next/Previous Tab" menu items and accelerators, also
  Programs menu now always displayed and has a "Console"
  menuitem too.
* Mixed fractions can now be set in the preference box
* The preference box has options for remembering (or forgetting)
  the output/precision settings.  Normally these settings are probably
  temporary and hence we don't want to remember them always.
* Preference box has "Help" button
* When graphing a surface when we encounter complex value or other errors,
  don't assume 0, but just don't graph anything at that point.
* Cosmetic changes to UI
* Documentation updates
* Fix plugin reading
* Portability fixes
* Translation updates (Pema Geyleg, David Lodge, Francisco Javier F. Serrador,
  Djihed Afifi, Pawan Chitrakar, et moi)