Add IsSublte and HorizontalAlignment to Element

[codello]

This PR adds two fields to the Element struct:

• IsSubtle: For setting subtle text appearance
• HorizontalAlignment: For setting the horizontal alignment of text elements

Reference: https://adaptivecards.io/explorer/TextBlock.html

I reused the horizontal alignment values from Column.HorizontalCellContentAlignment as the values are the same and I felt that having something like HorizontalTextAlignmentLeft would cause confusion when the field itself is called HorizontalAlignment.

Closes #254

[atc0005]

I reused the horizontal alignment values from Column.HorizontalCellContentAlignment as the values are the same and I felt that having something like HorizontalTextAlignmentLeft would cause confusion when the field itself is called HorizontalAlignment.

Since that helper function is unexported we could take a couple of approaches:

1. rename supportedHorizontalContentAlignmentValues to supportedHorizontalAlignmentValues
   • also update the doc comment to remove the word content since that was specific to the Table type
2. add a second helper function named something like supportedHorizontalTextAlignmentValues
   • this function could either call the other function (e.g., return supportedHorizontalAlignmentValues()) or duplicate the slice of constants
   • having this function might make clear that there is a distinction between TextBlock and Table alignment values (assuming there is a distinction)

I'm in favor of option 1 if we're looking to reduce duplication.

[codello]

rename supportedHorizontalContentAlignmentValues to supportedHorizontalAlignmentValues

I also prefer that option. I updated the PR.

[atc0005]

@codello Thanks!

Are you comfortable squashing your commits?

[codello]

Sure, done.

[atc0005]

Thank you for all of your work on this!