Fix triggerScrollStart() so that it can properly ignore prevention conditions

[yujinakayama]

The documentation comment for triggerScrollStart() describes:

Overrides any non-size condition which is preventing the receiver from automatically scrolling, and begins a scroll animation.
Currently the only non-size conditions which can prevent a label from scrolling are the tapToScroll and holdScrolling properties.

MarqueeLabel/Sources/MarqueeLabel.swift

Lines 1325 to 1327 in bfab54b

	Overrides any non-size condition which is preventing the receiver from automatically scrolling, and begins a scroll animation.
	Currently the only non-size conditions which can prevent a label from scrolling are the `tapToScroll` and `holdScrolling` properties. This

However, invoking triggerScrollStart() on a label with holdScrolling = true does not trigger scroll.
This PR fixes the issue by calling updateAndScroll() with overrideHold: true option.

[cbpowell]

Thanks for pointing this out! I'll take a look this afternoon and get it merged shortly.