Encode line breaks as <br> tags for file saving

[miiizen]

Resolves: #19189

This PR encodes line breaks as <br/> for TextBase items, fixing the loss of newlines after XML tags - eg. formatting & symbols.
This isn't a huge change and preserves backwards compatibility. TextBase still needs to turn \n into newlines when setting text on user input. This means old files using actual newlines in the XML can still be read.

[cbjeukendrup]

If we wanted to be less verbose and use a <br/> tag, we would need to implement a way to condense tags with no inner content in XmlReader::htmlToString.

That would be nice indeed... it looks like TinyXML's XMLElement::ClosingType could help here, but the bigger problem is of course that the interface of XmlStreamReader doesn't offer a way to differentiate between a start tag or a <…/> tag (they are both StartElement). And changing the meaning of StartElement in order to introduce a new enum value for <…/> tags, might be intensive. Perhaps m_xml->node provides enough information to make a new member in XmlStreamReader that just queries whether the current start tag was a <…> or a <…/>.

Also, this is the sort of thing where a unit test would seem nice to me (and perhaps even more appropriate than a VTest).

[mike-spa]

Uhm, I don't love the idea that the layout code knows about this, unless it's absolutely necessary. As far as I understand, the problem only involves the XML reading, not the program runtime. The program itself has no problem dealing with line breaks, so I don't see a reason why the substitution of line breaks with tags should be carried all the way into the runtime. If it's possible, I think the ideal approach would be

• On write: check the string for line breaks just before writing it to the file, and write to the file a version of that string where line breaks are replaced with tags.
• On read: read the string normally, then do a second pass on it to check it for line-break tags and replace them with actual line breaks.

That way the knowledge about this tag is strictly confined to the read/write modules and no other part of the program needs to deal with it.

[miiizen]

Sure, I think you're probably right - though couldn't this be extended to all other tags in the xml text?

[rpatters1]

I am hoping this PR will also work with instrument names. I frequently have instrument names like this:

MuseScore itself (I have discovered) can handle this if you programmatically insert the new line. But of course the new line gets lost when the file is read back from xml.

What's really needed (but obviously out of scope for this PR) is the ability to edit instrument names in the full text editor.