Doc: Added platform and porting notes for iOS

Discuss deployment and publishing to App Store for
Qt for iOS apps.

Add a brief porting guide, similar to the one for
Android.

Task-number: QTBUG-37490
Change-Id: I7e7ba4037694cf092dcff27811d841611a417d6b
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Doc: update scalability topic

Add info about Qt Quick Controls and Layouts,
attached Window and Screen property bindings,
file selectors, and loaders.

Change-Id: I50a90ab9b356dbe986980b15462eae9530000d2b
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>