xcodegenerator: use a copy resource phase if possible

Since commmit 0127962e

 the
PBXResourcesBuildPhase is used only for ICONS, because the old
behavior of using it when target path is not given differed from
the documentation and behavior of the makefile generator by using
Contents/Resources as target directory when targeting osx.

The PBXResouceBuildPhase optimizes png, compiles xib or asset catalogs
and copies the rest.
The advantage is that it makes it easy to add resources to the bundle,
the only problem is that the target directory is always the resource
directory.

The copy operation currently used does not compile resources, which
makes adding .xib (for the Launch File required to support iphone 6)
and asset catalogs difficult.

So we restore the old 5.3 behavior for ios, and use the build
resources phase when possible on osx (target Contents/Resources).

On osx this still implies a difference between the makefile
generator and the xcode generator: only the latter compiles resources.

Change-Id: Id1853693e88fc46562b044efdea2bf5f9da2c98c
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>