Markdown support

[Krzmbrzl]

Supported are sections, inline code and code-blocks, bold, italic and
strikethrough text, links and quotes.

It is also possible to use the backslash character to escape other
characters (to prevent them from triggering Markdown interpretation).

The changes made in this commit do not apply to direct messages though
(which are assembled via a RichTextEditor and I haven't found a clever
way to hack into that one yet).

This implements #2945 and fixes #4075

---

An example:

# Markdown
## Code
We can have `inline` code and
```code
  blocks```

## Emphasis
**bold**  and *italic*  text is also supported. ~~Scratch that~~

## Links
We can have [links](google.de) and www.google.de

## Quotes
> I am a quote
Although these are not pretty, they do work and are supported.

## Escaping
We can escaoe markdown using the \\ character: \*not italic\* and \*\*not bold\*\*

yields

---

Changelog

| Added: Markdown support for TextMessages

[Johni0702]

I believe it's generally expected that the zeroth line of code blocks (i.e. the remainder of the line after ```) be ignored (or used to specify the language). See e.g. GitHub's Markdown spec: https://github.github.com/gfm/#fenced-code-blocks

E.g.

```
test
```

should render as as single line:

test

not as two lines:

test

and

```java
test
```

should render just like the first example with optional syntax highlighting (I'd consider that part out of scope of this PR though), not as

java
test

[Kissaki]

How does this fix #4075? Markdown does not typically render " quoted text as italic.

[Kissaki]

I would definitely put the Markdown parsing logic into it’s own class.

TextMessage is a ui component, and Markdown conversion is quite a distinct functionality.

[Krzmbrzl]

@Kissaki the issue states that although the functionality for quotes is broken, it is good the way it is.

This PR has removed the respective code area altogether and provided proper support for quotations. That's why I think this closes the issue.

[bkacjios]

This is great! I was going to try my hand at doing this today, but I guess I don't have to now.

I only reported #4075 as an issue because I really didn't like the functionality, especially since it wasn't documented anywhere and there was no way to escape it. This implementation is more inline with what I'd expect, so I'm all for it.

[Krzmbrzl]

@Kissaki I moved all markdown-related code to its own file (no need for a class though)

@Johni0702 good catch! I adapted the code accordingly

[vimpostor]

BTW Qt already has builtin support for Markdown: https://doc-snapshots.qt.io/qt5-5.14/qtextedit.html

[Krzmbrzl]

BTW Qt already has builtin support for Markdown: https://doc-snapshots.qt.io/qt5-5.14/qtextedit.html

Yes but only from 5.14 onwards. Until this is the version that is supported by the oldest still supported Ubuntu release, it'll probably take quite some time. And before that we can't make use of it.

[Krzmbrzl]

As a reference: Qt's implementation of Markdown stuff seems to live in https://github.com/qt/qtbase/tree/5.14.0/src/gui/text

[fedetft]

Hi, I tested this new feature, and it's great!

However, I think I found a bug, the < and > characters in markdown code blocks get replaced by &lt; and &gt;

[Kissaki]

@fedetft thank you for reporting back. Please create a new bug ticket for it as this merge request is already completed. Issues found are new bugs now.