This is a follow-up to bug #417446 in order to deal with the remaining TODOs in the multipress IM code.

> /* TODO: The delete key does not seem to be handled when we do this.
>  * danielk: Yes, that's normal and it's not the only one.  I'll get to
>  * that later. */

Delete and other control keys are mishandled because the text input widget handles them before the IM filter receives the corresponding key release event.  As it turns out the problem can be avoided by simply filtering key press events instead.  This is the more natural behavior anyway, and the special casing already in place for some control codes can be dropped, too.