Revise main() in examples

Replace the inconvenient QT_QUICK_CONTROLS_EXAMPLE_MAIN macro with a
simpler macro that merely defines the actual application type. This way
it's easier to do tweaks to main() when for example testing a different
application font size. Moreover, browsing the example code becomes more
convenient (especially online).

Task-number: QTBUG-35083
Change-Id: I8203816a9112bf29b4661baa86bf4a196554db05
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>