[Enhancement] FormattedText should have an implicit converter from IList<Span>

[softlion]

Summary

Currently to use the FormattedText property you need to add a FormattedString tag:

<Label>
<Label.FormattedText>
    <FormattedString>
         <Span Text="Some text" />
         <Span Text=": " />
     </FormattedString>
</Label.FormattedText>
</Label>

This is a lot of verbosity that can be shrinked easily by adding a TypeConverter on the FormattedText property of the Label, which would converts an IList<Span> into a FormattedString.

The simplification will leads to this more readable and much less verbose xaml:

<Label>
<Label.FormattedText>
     <Span Text="Some text" />
     <Span Text=": " />
</Label.FormattedText>
</Label>

To go further, the default Content property on the Label could be set to FormattedText leading to this xaml:

<Label>
   <Span Text="Some text" />
   <Span Text=": " />
</Label>

API Changes

none

[BrayanKhosravian]

How would this be implemented?
The ContentProperty of the Label is already set to the Label.Text property which is of type string.
As far as I know, it is not possible to add another ContentPropertyAttribute to the label class.

I can currently think of 3 solutions:

1. Use a converter which accepts string or IList<Span>.
2. Make it possible to define more then one ContentProperyAttribute.
   This would mean that the identification of which ConentProperty is used has to be emitted and applied during runtime.
3. Add a new "MultiContentPropertyAttribute" which accepts multiple properties and selects the property which is used.
   This would result in extending existing code but not modifying it.

[softlion]

Hello !

The ContentProperty of the Label could be an object instead of a string. So it can be either a string or a list of spans.
A converter on it would make the decision.