gui: allow listbox to be used as menu and as read-only list

Also enable string insertion for list items.

Example how to make a menu item:
<listitem name="Lights on!">
	<action>
		<action function="setbrightness">255</action>
	</action>
</listitem>

If no <data> element and no actions on any items exist, the list is
read only and no item selection is possible.

Change-Id: Ib2668a982df2514484d44faa0396dd17550f39f3