Update spin value periodically if left mouse pressed is been holding

[Kvaz1r]

I'm not sure that Timer::scheduleCallback is the right choice here, but idea is simple - move changing value from mouse release handler to mouse pressed handler and to call changer periodically while some conditions (mouse still pressed, mouse still hover on widget and correct spin button and there wasn't any new clicks) are still being true.

[texus]

I like the idea of using Timer::scheduleCallback.
Although it is unlikely to happen, I fear that it could cause issues if the widget is destroyed between scheduling the callback and receiving it. Instead of capturing this in the labmda, it would probably be better to capture something like spinButton=shared_from_this() and add spinButton-> in front of variables and functions inside the lambda. That way the widget can't be destroyed until after the scheduled lambda has been executed.

[Kvaz1r]

Fixed

[texus]

Manually merged as 8e8db09