WinRT: Provide qmake feature for generating a package manifest

This feature (package_manifest) generates a basic application manifest
from a template provided by the mkspec or the developer. It is meant to
deliver an out-of-the-box build experience without attempting to
exhaustively cover all manifest options. It is meant to be a starting
point which allows the developer to customize the manifest further. It
also becomes the default package manifest generator for Windows Phone,
replacing autogen_wmappmanifest.

Common variables, such as the target executable, are populated by qmake
in the newly created manifest. Default icons are also created if needed,
as the build will fail without them. The input manifest can be set by
assigning a file name to WINRT_MANIFEST. Additional options are
documented in the .prf file. If an existing (non-generated) manifest is
already in the directory, it will not be overwritten.

Task-number: QTBUG-35328

Change-Id: I57576a17ff9d2b564c0828f815949cb26d276bfd
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>