Properly escapes in replies and with unmatched [].

fixes #854

I think old clang-format is just buggy in those cases, so we should
reenable those blocks, once everyone has 13 or up.

fixes #422

Fixes #234

Parsing html as xml has inherent problems, most notable there are many
matrix clients that don't escape ampersands in urls of mx-replies, etc.
(See issue #18)

This also removes the replacement of <mx-reply> as it isn't strictly
needed.

Also the QRegExp is replaced with the Qt5 QRegularExpression for
perfomance and because it supports lookahead and lookbehind.

I'm pretty sure that the original code also replaced href="" with
href=\"\", which was probably wrong, but I'm not to sure about that.

Fixes #18

The room list would be hidden only through window resizing.

fixes #235

fixes #204
fixes #255

fixes #40

fixes #157