Add an IsVisible Property to <Span> Elements

[david-maw]

Description

It is occasionally handy to compose a string from pieces, for example adding a parenthesized count of items when there are more than one.

You can do it with a HorizontalStackLayout containing multiple Label objects but that seems like overkill and requires manual kerning.

Public API Changes

Add a new bindable property IsVisible to the Span class.

Intended Use-Case

For the example described above in XAML

        <Label>
            <Label.FormattedText>
                <FormattedString>
                    <Span Text="{Binding ThingName}"/>
                    <Span Text="{Binding ThingCount, StringFormat=' ({0})'}, IsVisible={Binding IsManyThings}/>
                </FormattedString>
            </Label.FormattedText>
        </Label>