Add SizeName property to Hyperlink widget

[dweymouth]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

Currently it is not possible to have Hyperlink widgets of different sizes. This can be desirable in certain cases including Markdown (a header that is also a link), or in something like a music player app where an album title that is a hyperlink may want to be larger than the artist, etc.

Is it possible to construct a solution with the existing API?

No, without rebuilding the Hyperlink widget from scratch. Can't do it with RichText because RichText doesn't handle underline - the underline is drawn customly by Hyperlink.

Describe the solution you'd like to see.

Add a SizeName property to Hyperlink that is a theme size name for the text size. (eg Subheading)

[dweymouth]

On develop for 2.5