Core: Move QMetaSequence docs and implementation to own file

This is in preparation for adding further inner classes to QMetaSequence
that can't live in qmetacontainer.{h|cpp}. Also fix bogus references to
QMetaSequence in QMetaContainer.

Pick-to: 6.8
Task-number: QTBUG-140181
Change-Id: Ieed1a3766c7d9f930eac08e6c132e3b664a97ba7
Reviewed-by: Volker Hilsheimer <[email protected]>
(cherry picked from commit bc8dee1)
Reviewed-by: Fabian Kosmale <[email protected]>

Author: ulfhermannqt

src/corelib/CMakeLists.txt

@@ -175,6 +175,7 @@ qt_internal_add_module(Core
         kernel/qmetaobject.cpp kernel/qmetaobject.h kernel/qmetaobject_p.h
         kernel/qmetaobject_moc_p.h
         kernel/qmetaobjectbuilder.cpp kernel/qmetaobjectbuilder_p.h
+        kernel/qmetasequence.cpp
         kernel/qmetatype.cpp kernel/qmetatype.h kernel/qmetatype_p.h
         kernel/qmimedata.cpp kernel/qmimedata.h
         kernel/qtmetamacros.h kernel/qtmocconstants.h kernel/qtmochelpers.h