if-then-else for <check>

[malcolmh]

The current <check> only allows one key with one of two values. Often it is required to select alternative key/value pairs, rather than alternative values for a single key. Also more than one key/value pair may be wanted. I suggest the following enhancement to <check>:

<check text="..." default="...">
  <check_on>
    <key key="..." value="..."/>
    <key ... />
  </check_on>
  <check_off>
    <key key="..." value="..."/>
    <key ... />
  </check_off>
</check>

Where only one key/value pair is required, then <check_on key="..." value="..."/> or <check_off key="..." value="..."/> would be used for either or both the then & else cases.