When generating the preview, only the first 128 bytes of the first MIME
part is fetched and used. If this part is text/html with a significant
amount of embedded CSS, then there is a good chance the string passed to
Geary.HTML::remove_html_tags() will be invalid, or be missing closing
elements. Since that function uses regexes that require balanced tags to
remove script and style blocks, then it was very possible that in these
cases this method will miss removing these blocks.
To solve this, remove_html_tags() is removed and its call sites are
replaced by calls to Geary.HTML::html_to_text(), which has been tidyied
up to produce more human-readable result.
Add unit tests to cover new html_to_text functionality and its call
sites.
* src/engine/util/util-html.vala: Remove remove_html_tags(). Update
html_to_text() to not just insert line breaks, but also insert spaces
and alt text, and ignore tags like HEAD, SCRIPT and STYLE, as
appropriate. Add an optional param to also allow skipping BLOCKQUOTE
elements, which we don't want in the preview.
