Use xterm-style cursor for Konsole version 18.08 and later.

[drmikehenry]

As of Konsole version 18.08, xterm-style cursor escape sequences are supported[^1]. If KONSOLE_VERSION set set to 180800 or later, use xterm-style sequences instead of cursorshape sequences for Konsole; this allows togglecursor to work correctly when tmux is used under Konsole while TERM is set to xterm or similar (the default Konsole configuration on many systems).

[^1]: "Add support for changing cursor shape with extended DECSCUSR escape sequences": https://bugs.kde.org/show_bug.cgi?id=347323