Add the option to switch on/off the display of variations

[ebifrier]

This enables to switch on/off the variasions display started by mouse over.
It corresponds to the last comment in #649 .

[yishn]

I think what @SHKD13 wanted was the ability hide the heatmap completely. I don't see any reason why we would disable playing out variations on mouse over. Just don't mouse over if you don't want to see the variation played out.

[ebifrier]

What you say is theoretically correct. However, it is difficult for a careless person like me to always be careful not to hover over. I'm sure that this option is useful for many people.

[yishn]

Then I would prefer to merge this option into the Preferences drawer along with the setting 'Instantly play out analysis variations on board'. We can have a select box instead with the options 'Don't Play', 'Instantly', and 'Move by Move' or something like that.

[ebifrier]

Ok. That's a nice idea! I'll try to fix it.

[ebifrier]

I added the new variable 'board.variation_replay_mode' to the setting, and removed 'board.variation_instant_replay'. Please review it.

Also, I set the title of the preference item to 'Replay mode of analysis variations:', it may be a bit weird. Please let me know if there is an alternative.

[yishn]

Looks very good, thank you so much!

[ebifrier]

Thank you for your reply! Thanks a lot!