Fixed problem on combobox with search limit.
    
    The IPA.combobox_widget has been modified such that if the drop-down
    list doesn't contain the stored value (due to search limit) it will
    not select anything from the list.
    
    The widget has also been modified not to select the value that matches
    the filter automatically because that might not be the user's intention.
    
    Ticket #1819