tsm: vte: send original VT220 escape codes for Shift+F1 to F12

tsm: vte: send original VT220 escape codes for Shift+F1 to F12

Nearly all terminals behave differently regarding Shift+FX keys. We now
mimic the behavior of the classic VT220 instead of the new-style function
keys. Nearly all applications expect the old codes.

This might be changed in the future, though. It does make much more sense
to send the new codes as they provide more information to the application.
And remappings should be done via XKB instead of inside of kmscon.
However, as long as xkbcommon does not have a config-file, we will be
stuck with this little hack.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>