examples/gui: Add asserts for automated widget testing

* examples/gui/create_dataviewlist_ex1: Re-enable helper widget method.
* examples/gui: Re-arrange the rendering positions for 'button_ex0',
'dataviewlist_ex1', 'dataviewlist_ex2' && 'question_dialogbox_ex0'
widgets.

* Implement asserts for automated testing of each widget's position,
size, label texts and so forth. (Replaces dumping of said
information per widget).

* DataViewList: Implement getter methods from DataViewListStore --
column(int), columns_size(void), item(int, int), items_size(int) --
for our assert automated widget testing in examples/gui.cpp.