[tin 1.7.x] 'unbinding' keys

one can not 'unbind' the actions from the defauilt keys, e.g.
ShellEscape	"
in .tin/keymap still allows the default of '!' as ShellEscape key.
as ppl. might like disable certain functions (e.g. QuitTin or
SelectQuitNoWrite) by binding them to a 'safer' key (like '^') this
isn't very usefull. we might also need some wey of completly disable
certain functions by bimnding them to an 'empty' key like:
QuitTin		EMPTY

comments? code? ,-)

urs
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