Enable snapping to PeriodStart and PeriodEnd as well, when IsSnapToIntervalEnabled is set to "true"

Add the ability to extend/contract the current selection by Shift-clicking. This can be used along with the "IsSnapToIntervalEnabled" property.

If the PeriodStart and VisiblePeriodStart is set to DateTime.MinValue zooming out using mouse wheel causes a crash. DateTime.Subract(TimeSpan value) method throws "ArgumentOutOfRangeException();". Looks like "TimeBarBase.CalculateLeftRightStep(this.SmallChange, centerOffsetPercentage, out leftStep, out rightStep);" does not calculate leftStep correctly.

The values of VisiblePeriodStart and VisiblePeriodEnd properties are not respected after runtime theme change.