Some analysis done with Callgrind showed that the `LcdWidget` spends quite some time repainting itself even when nothing has changed. Some widget instances are used in song update contexts where `LcdWidget::setValue` is called 60 times per second.
This commit fixes the problem by only updating the `LcdWidget` if its value really has changed.
Adjust the condition in the if-clause so that it becomes clearer what's the interval of interest.
5d5d8f8f14