Set "mixed" to the preferred key type

Webform uses the "mixed" option in its usage of Options Element. Mixed is the format that is most compatible with existing paradigms in CCK, Profile, Webform and other modules. It allows mixed keyed and non-keyed items such as:

One
Two
three|Three
four|Four

This would result in a set of options such as:

array(
 0 => 'one',
 1 => 'two',
 'three' => 'three',
 'four' => 'four',
);

Currently "associative" is the default type, which is limited because it does not support keys in any way. Associative only supports structures such as:

One
Two
Three
Four

Which would result in an array like this:

array(
 'one' => 'one',
 'two' => 'two',
 'three' => 'three',
 'four' => 'four',
);

Because "mixed" is much more useful in determining actual useful keys and can be toggled between custom keys and non-custom keys, we should use it as the default. Associative does not allow keys to be custom edited, because they always get overridden with the value.