Bugfix for calculate score for evalbar in engine window

[ukscid]

If evaluate from engine is mate in X, then evalbar was called with "+MX" instead of a float number

[benini]

That's no good. ::notify::EngineBestMove should be invoked with the same displayed value because it may be shown somewhere else (for example chess.com also shows it in the bar). And this is horrible:

• string map is invoked even if it is not necessary and we do not enter the if.
• it is not possible for the recipient of the message to distinguish between +M2 and +12
• the order is reversed: +M2 becomes a worse evaluation than +M3