Improve qtquick controls page

Added more information for "Getting Started"
- Cleaned up some language
- Remove desktop specific
- Mention widget dependency
- Use QmlApplicationEngine in example

Change-Id: I0e5d4bef124ce4571c1b6fe780ee78b8231cbb32
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Prevent possible division by zero

This might cause problems on certain platforms. While hard
to reproduce it seems safer to add the check.

Note that we do not get the alternating pattern if there is nothing
set on the model, but this was already the case.

Change-Id: I984df9cfef30b4de0e72e8ccf14cc65bddbf04ef
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>