Webclient: aggregated numbers (#588, #240).

The aggregation semantics is now implemented for Number types as well.
Aggregated number instances are created only once (#588).
The user can select/enter an aggregated number via a combo box (#240).

GUIToolkit API:
    Class Combobox has a new method:
        - val()

RenderHelper API:
    The dm4c.render object has 3 new methods:
        - form_element(page_model, parent_element)
        - form_element_function(form_element)
        - get_option_topics(page_model)

Close #588.
Close #240.