Xcode: Make QMAKE_BUNDLE_DATA always copy files relative to root of bundle

Instead of sometimes ending up inside Content/Resources. The two build
phases PBXCopyFilesBuildPhase and PBXResourcesBuildPhase have different
semantics of where to place the files. For the former we use the root of
the bundle as the destination, and this is how QMAKE_BUNDLE_DATA is
documented and used, as well as how unixmake2.cpp implements it. The
latter on the other hand, always ends up in the resources subdirectory
on OSX.

Task-number: QTBUG-35318
Change-Id: I45bbd0dfe7ea78ae330ecb0c91efa74e1c76c9eb
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>