Minor improvements to memory representation in calculation history

Fine-tuning: reduce button height by 4px

ViCalc Overview help topic added. Bug-fixing.

Fine-tuning: filter clipboard paste input to +-.,0123456789eE

Added help topic "Basic operations"

Fine-tuning Undo/Redo Ctrl+Z/Ctrl+Y

Bug-fixing Delete key

Fine-tuning: pi and M+ operations swapped on left side

Initialize pytest

Bug-fixing