ui.slider - moveTo assumes string-value is always relative

In the current moveTo implementation, if the value is a string, its always handled as a relative value. Instead it should be checked if the value really starts with +=, and applying the relative stuff only then.

This works quite well:

if(value.constructor == String) {
	if (/^\\-\\=/.test(value) ) {
		value = this.value() - parseInt(value.replace('-=', ''), 10)
	} else if (/^\\+\\=/.test(value) ) {
		value = this.value() + parseInt(value.replace('+=', ''), 10)
	} else {
		value = parseInt(value, 10);
	}
}