Xcode: Fix storyboard view

Since commit 56831461 (Xcode: Use explicitFileType to mark source types,
2013-04-16) the Xcode generator prefers to use explicitFileType to tell
Xcode about each source file type.  This works better than
lastKnownFileType for some file types, but not for "file.storyboard".

If storyboard file has attribute 'explicitFileType' it is displayed
incorrectly (as raw xml).  Switch it back to 'lastKnownFileType'.
Xcode: Fix storyboard view

Since commit 56831461 (Xcode: Use explicitFileType to mark source types,
2013-04-16) the Xcode generator prefers to use explicitFileType to tell
Xcode about each source file type.  This works better than
lastKnownFileType for some file types, but not for "file.storyboard".

If storyboard file has attribute 'explicitFileType' it is displayed
incorrectly (as raw xml).  Switch it back to 'lastKnownFileType'.
Merge branch 'xcode-storyboard-file-type-2.8.12' into xcode-storyboard-file-type

Resolve conflict in Source/cmGlobalXCodeGenerator.cxx by keeping
our side.
Merge topic 'xcode-storyboard-file-type' into next

e361c3c Merge branch 'xcode-storyboard-file-type-2.8.12' into xcode-storyboard-file-type
93fc5a5 Xcode: Fix storyboard view
2d23c1f Xcode: Fix storyboard view