Previously the `<del>` tag that Discount emits would be removed by Qt
since it's not in the subset of HTML that it understands. It does know
about `<s>` though, so replace `<del>` with `<s>` to render strikethroughs
correctly.
Note that I couldn't get Okular to build locally (PEBCAK issue) so I split out this file to debug it. I've tested against http://daringfireball.net/projects/downloads/MarkdownTest_1.0.zip and ff866f73d4/tests/data and the only difference is that trailing whitespace is now trimmed from the end of a line:
```diff
8c8
< <p style=" margin-top:12px; margin-bottom:12px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;">Dashes:</p>
---
> <p style=" margin-top:12px; margin-bottom:12px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;">Dashes: </p>
```
A quick search suggests this isn't a problem though https://stackoverflow.com/a/27026403:
> Within block tags (i.e. `<p>`, `<h1>`, `<div>`, ...) spaces as well as line breaks at the beginning or end of the tags should always be ignored (i.e. `<p>test</p>` should look the same as `<p> test </p>`).
I also tried https://doc.qt.io/qt-5/qtextdocument.html#setMarkdown which solves the strikethrough problem, though there are many visible differences with that approach. Most notably, everything is squashed together and code blocks lose their formatting.
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