Centralise generation of the control sequences for arrow keys into a

Centralise generation of the control sequences for arrow keys into a
function in terminal.c, and replace the cloned-and-hacked handling
code in all our front ends with calls to that.

This was intended for code cleanliness, but a side effect is to make
the GTK arrow-key handling support disabling of application cursor
key mode in the Features panel. Previously that checkbox was
accidentally ignored, and nobody seems to have noticed before!

[originally from svn r8896]