Allow Esc/Delete/Backspace for Unbinds

Modifies the Conf_keyPressEvent handler such that if the user presses
either the Escape, Delete, or Backspace keys, the keyval will be stored
as None.
Update to README

Expands README to mention that Backspace and Delete will also allow the
user to unbind a keyboard shortcut.
Display Unbound Shortcuts as "Unassigned"

This adjusts the KeyToString function to return "Unassigned" when keyval
has no value (rather than "Unknown", which can still be reserved for
when there is a value, but the system cannot tell what it is).
Don't Repeat Yourself for Conf_keyPressEvent

Adjusts the Conf_keyPressEvent logic such that it gets its field label
from the return of KeyToString instead of having its own version of the
label.