Rewrite the gallery example

A much simpler, scalable and mobile friendly UI.

Some of the more complex controls, such as TextArea and TableView,
are intentionally left out. They are scrollable on their own, so
they wouldn't play nice inside scrollable pages. Furthermore, adding
them as standlone pages would increase the amount of tabs too much.
Those controls are demonstrated in the Text Editor and Table View
examples, respectively.

Task-number: QTBUG-41307
Change-Id: Ib3efb346e621e388087f3f14c73b3b54279db280
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@theqtcompany.com>