Add attached property to allow slider tool tip formatting

[cjmurph]

This feature can work around issue #2685 and it's also useful to have.

The attached property allows a string format to be spcified as per below:

    <Slider  
        wpf:SliderAssist.ToolTipFormat="a value of {0} is selected"
        Maximum="100"
        Minimum="0"
        Style="{StaticResource MaterialDesignDiscreteSlider}"
        TickFrequency="10"
        TickPlacement="BottomRight"
        Value="{Binding Value}"/>

Which look ssomething like this

With nothing specified or an invalid format, the value is shown raw and unformatted.