Windows: Add an option to build without wxMaxima, update mswebview.
Windows installer: update SBCL.
Website: Add Maxima 5.50.0 to de/versions.html.
Windows: Set ECL_ENABLED correct (0 when not enabled).
Windows installer (with ECL): Update w64devkit.
Windows installer: Build wxMaxima without libfribidi requirement.
Windows installer: Update Gnuplot.
Windows installer: Update mswebview.