Feature Request - add optional centre candidates

[wyzelli]

Add the option for the hotkey '.' to switch the selected cell candidates to centre mode whilst using Simple Candidate mode.

This enables the pattern recognition of simple candidate mode, as well as the visual cue that a pair exists when using centre candidates. I suspect this should not be default behaviour but be an option which can be enabled by choice.

If the user adds new candidates to a cell already switched, then those candidates can follow the new scheme, though adding new candidates would be an incorrect move, since the centre mode would already indicate a pair (in normal usage).