Implemented Speech.DisplayPostTimeMs script property

Speech.DisplayPostTimeMs property defines an additional period of time for the speech text and character's speech view to be displayed after main speech timer has run out (or voice clip finished).

Also:
* fixed game.close_mouth_end_speech_time had effect in voice mode with "Sierra" speech style (athough it shouldn't have);
+ deprecated game.close_mouth_end_speech_time script variable and introduced Speech.AnimationStopTimeMargin property to be used instead.