* Made XDatabaseContext inherit XDatabaseRegistrations non-optionally, adapted
  call-sites to just use XDatabaseContext w/o querying.  (The previous commit
  had inadvertantly effectively removed support for XDatabaseRegistrations from
  the ODatabaseContext implementation, as an optional UNO super-interface does
  not lead to a super-class in the corresponding C++ class hierarchy, but making
  the super-interface non-optional fixes that anyway.)

* Adapted some more call-sites to just use XDatabaseContext w/o querying.

* Added @since tag.

* Replaced new uses of comphelper::ComponentContext::getUNOContext with
  comphelper::getComponentContext (see 03a9f139
  "ComponentContext::getUnoContext -> getComponentContext simplification;" I
  intend to get rid of comphelper/componentcontext.hxx much sooner than of
  comphelper/processfactory.hxx).

Change-Id: I68d09f2dbe651629f79ed21cd40cdb6d6b32c624