Fix OSC sequence parsing

Fix for https://bugs.kde.org/show_bug.cgi?id=231405

• printf '\033]2;aaa\033bbb' prints bbb
• printf '\033]2;aaa\033\\bbb' prints bbb
• printf '\033]2;aaa\033' prints nothing; title is changed when next character is received.
• printf '\033]2;aaa\033\\' prints nothing
• printf '\033]2;aaa\007bbb' prints bbb
• printf '\033]2;aaa\007\\bbb' prints \bbb
• printf '\033]2;a\ra\na\033bbb' prints bbb
• printf '\033]2;a\ra\na\033\\bbb' prints bbb
• printf '\033]2;a\ra\na\007bbb' prints bbb In each case the title is set to aaa