Add markdown support

[krombel]

especially when writing

[rushsteve1]

It looks like most Matrix clients render the Markdown to HTML on the clientside then send it as an HTML message.
So we would probably need a Markdown library. Nheko uses Cmark which is probably the best choice for us as well.

[KitsuneRal]

What if we use basic RTF support that Qt already has? Qt can also easily convert it to HTML but unfortunately the set is richer (and uses CSS) than that supported by Riot.

[rushsteve1]

I did some experimenting with the RTF in QT and while I still think that isn't a bad way to go, it's not the best. The big issue with it is that when it generates HTML it makes a full valid HTML document when we need just a a smaller portion.

That mixed with trying to only expose a subset of the RTF functionality to users makes it rather hard to use.

So I am thinking it would be best if we either found a Markdown library to use (as much as I hate adding dependencies) or wrote our own parser for a Markdown subset (also not a great idea).

[rushsteve1]

I have experimental Markdown support working on my fork: https://github.com/rushsteve1/Quaternion/tree/cmark

It uses CommonMark which... To be honest isn't great. It doesn't support some common features such as underline and strikethrough. So if someone knows of a better library I could switch it.

[KitsuneRal]

So far I feel that functionality of Markdown is so small that integrating another library may bring more overheads than benefits, compared to the homebaked code. Just an observation.

[KitsuneRal]

Qt 5.14+ supports Markdown; #701 has a proof-of-concept in the form of /md command (assuming you build with recent enough Qt).

[KitsuneRal]

As of 0.0.9.5, there's /md command now; there's also going to be a hidden setting to make markdown the primary entry notation (instead of plain text+HTML). If this mode proves useful, 0.0.9.6 will have it exposed in settings.