Xcode: Make sure QMAKE_PRE_LINK dependencies are complete for multi-arch builds

With multi-architecture builds and ONLY_ACTIVE_ARCH set to NO, Xcode will
build the final target for multiple architectures at the same time, but
CURRENT_ARCH will only match one of them, so we failed to set up the
right dependencies for our pre-link step, causing the step to happen
after linking in some cases.

We now build an exhaustive dependency list based on QMAKE_XCODE_ARCHS,
so that ONLY_ACTIVE_ARCH=NO can be used for release builds targeted at
the App Store.

Change-Id: I6702f020a6970807adc624779f6dde09be62beb9
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
iOS: Remove assert when transferring or clearing first-responder

We'd transfer or clear first-responder in a lot more cases than just
when transferring to a new Qt window, such as when presenting a new
view-controller on top to send an e-mail or take a picture using the
camera.

Change-Id: I6b2a8a6d9fd99910b96a86cf9847b7ff0128f20a
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>
iOS: QtFirstResponderEvent needs to release firstResponder to avoid leak

Without setting firstResponder to 0 upon destruction, the current retain
count would never reach zero after the event was used. The result being
that QIOSTextResponder was seldom destroyed, which would also affect its
inputView etc which would also be kept alive.

Change-Id: Ia88e6a9d8764e7e9532487153e5e81a7ad0f9741
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>