Add AppendMarkdown function to RichText

[dweymouth]

Description:

Allows appending additional markdown text to an existing RichText

Fixes #4866

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

Where applicable:

• Public APIs match existing style and have Since: line.

[coveralls]

coverage: 65.571% (+0.5%) from 65.043%
when pulling 6672d89 on dweymouth:richtext-appendmarkdown
into 9810934 on fyne-io:develop.

[andydotxyz]

This carries potential risks which we might need to think about or document.
For example a markdown document should only have one title (h1) - but with this addition you could have any number.

Also the interpretation of markdown may depend on what came before it.
I.e. this function cannot be used for streaming multiple chunks of a markdown file - unless it is carefully parsed first. Possibly it's only useful for concatenation of documents?

[dweymouth]

Concatenation of documents - or rather fragments of text that might have markdown elements like lists but aren't full documents per se - was the use case from Discord that made me think of this feature addition. Is there really anything preventing a markdown text from having multiple titles anyway?

# Title 1

### subsubheading
* some stuff

# Title 2

I agree though that the API is not intended for piecewise handling of a single markdown document - maybe it needs a naming update to reflect that? AppendMarkdownDocument?

[andydotxyz]

Yes you can technically have multiple headings, it's the spec that disagrees.

Not sure if it's an API naming issue or just documentation. "ParseMarkdown" also requires that it be a full document so we shouldn't need to introduce new naming to reflect that.

I agree that fragments are ok, but they do need to be complete in their own right. Maybe that's what the doc needs?

[dweymouth]

Updated API doc to include the caveat against processing a single markdown document piecewise.

[andydotxyz]

I don't think we should encourage piece-wise parsing in any way as it's not supported.

[andydotxyz]

The indication that "ParseMarkdown" can work piecewise doesn't seem correct - that code is being used within this helper so I don't know how it could be true.

[andydotxyz]

Thanks, I'm sure this will be helpful