Autocomplete fires on readonly fields

Issue Summary

If you have a readonly textfield with an autocomplete path, the autocomplete request fires when the user places the cursor in the field and presses any key, even though the textfield is readonly.

The use case for this is to have an autocomplete textfield that you want to enable through some other means (checkbox or other field filled in beforehand). In that case, you don't want the autocomplete to fire until the field is actually writeable.

I have attached an SVN patch with the hack that we will include on our platform, but since it uses jQuery() directly in the autocomplete's prototype context, I really don't think it is the best solution - but it's the best that I could come up with at that point in the code - I could perhaps have done something within the populatePopup() method itself, but I feel that it belongs with the check for input.value.length > 0

I would love it, if someone better at javascript/prototype/autocomplete could come up with a better solution.