Escape special characters (&, <, >, ", ') in the title attribute of
TextThreadContext's XML output to prevent malformed XML when titles
contain these characters.

Resolves TODO at context.rs:629

Release Notes:

- N/A

Signed-off-by: Xiaobo Liu <cppcoffee@gmail.com>
Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>