Add Underline support to Label/RichText

[dweymouth]

Related: #4297

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?

It would be nice to support an underlining style for text.

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

No. Other than drawing a canvas.Rectangle along the text baseline (which is how it could be implemented internally)

Describe the solution you'd like to see.

Underline bool added as a property to fyne.TextStyle

[GaetanBaert]

Underline seems implemented in #4244 for TextGrid, but not for Text though.

[dtrehas]

You could also add a wavy and strike through line for dictionary checking and text deletion accordingly.

[andydotxyz]

Please add new requests in a new issue. Underline is a substantially easier implementation than wavy. Additionally strikethrough requires APIs we don't have internally as the position of that line is not trivial.

However also consider that the wavy underline is not part of any standard font style definition so you may actually have to right some font handling code if you're adding a spell checker.