applets/calculator: make arrow key navigation more predictable

The calculator applet allows arrow key navigation, but the base representation is included in this (reachable from four different buttons) where the user can't really do anything. For the other buttons. moving outside the grid uses the representation's navigation bindings as the default, which sometimes works but in others makes large unpredictable jumps.

Instead, we explicitly set predictable orders for moving beyond the edge of the field, using Pacman-style navigation (enters on the opposite side of the same row or column).