Add "trigger" type

[prabhasp]

commcare and some other jr-xform tools (like touchforms) don't make the readonly=true tag within a string-input (ie, what notes are currently compiled into) hide the entry box, and suggest that trigger is the right element to use.

To close this issue, note should still compile as-is; and we'll add a new trigger type will compile into:

<trigger ref="sometext">
            <label ref="jr:itext('label_prompt')"></label>
</trigger>