0029822: Make TDocStd_Document extensible

0029822: Make TDocStd_Document extensible

Two virtual methods NewDocument() and InitDocument() are moved from TDocStd_Application to its parent class CDF_Application. In TDocStd_Application these methods remain redefined. These little changes allow creation of a new document only in one virtual method NewDocument(). The methods CreateDocument() in all retrieval drivers are deleted.

Modified files:
- CDF_Application.hxx and cxx: two virtual methods NewDocument() and InitDocument() are moved from TDocStd_Application. The input parameter TDocStd_Document is changed to parent class CDM_Document.
- TDocStd_Application.hxx and cxx: redefines new virtual methods NewDocument() and InitDocument() of the parent class CDF_Application.
- BinLDrivers_DocumentRetrievalDriver.hxx and cxx, StdLDrivers_DocumentRetrievalDriver.hxx and cxx, XmlLDrivers_DocumentRetrievalDriver.hxx and cxx, PCDM_Reader.hxx: a virtual method CreateDocument() is deleted.
- TObj_Application.cxx, XCAFApp_Application.hxx and cxx: down-casting to a descendant class TDocStd_Document is applied.

Documentation:
- upgrade.md is modified.