math/wxmaxima: Update to 20.04.0

• Resolved a crash on right-clicking a GroupCell.
• View/Invert worksheet brightness now toggles an instant dark worksheet mode.
• Removed an unnecessary info dialogue on startup.
• Right-click-suggestions for similar command names.
• Corrected the initial slideshow size.
• A more stable language selection mechanism.
• An Ukrainian translation update.
• A Russian translation update.
• Add a German manual page.
• Help works again in the German translation.
• Extended the "fitting data" example.
• An example on how to iterate through lists efficiently.
• Many spelling corrections and better formulations.
• A big number of small bug-fixes
• A noticeable speed up.

Approved by: gerald (mentor)
Differential Revision: https://reviews.freebsd.org/D24573