Added class provides text font formatting with HTML font tags
Supported stuff:
- nested formatting;
- several text pieces formatted with the same formatting style at the
one message;
- styling applies only if non-whitespace symbol follows opening
formatting symbol and also non-whitespace symbol preceds closing
symbol;
- only multiline code font formatting supports new line inside of
message text.
Also fix#3804
This commit removes historykeeper and code only used by it.
BREAKING CHANGE:
qTox can't open old historykeeper chatlogs after this commit. Use
previous versions to migrate to the new database.
