Fix assert crash in the composer when settings are reloaded

Looks like the settings shall be changed to not reload stuff that aggresively...
HTML: refactor the plaintext -> HTML convertor

This will make it much easier to introduce the quote collapsing in future
HTML: defer HTML escaping & formatting until later

We will need a preview of the actual raw line data without any fancy HTML stuff
for its inclusion in the shortened quote.
wip: decent approach towards the quote collapsing
wip: interactive collapsing with an appropriate short summary display now works
Don't collapse stuff where there's no difference between the expanded and collapsed text
Prevent line breaks when messing with the CSS pseudo-elements

Thanks to Ondra Machulda for a helpful suggestion.
webkit: adjust the window size in response to content size changes

This is necessary when e.g. the page height is to increase, typically in result
to user expanding a quote which was previously collapsed.
Fix a segfault
Simplify the code
HTML: use the correct level of <blockquote> nesting even for quote levels which are superfluous
Don't assert crash on a transition between ">> foo" and "bar" blocks
Tweak the heuristic on when to collapse and when not
Make it clear when the collapsed text also contains nested quotes
Make the quote collapser black and white
HTML: disable debugging
Remove excess debugging
tests: update the test suite for the changed quote collapsing thing
Updates for the quoting

- Simplify and rework the preview extraction; now it shall better pick "what is interesting"
- When working with non-flowed mails, be sure to use correct quotemarks for all subsequent lines as well
- Refactor the tests and add a few more test cases
More consistent heuristics which trigger collapsing