Add API in texteditor to let plugins contribute extra selections.

Some plugins use extra selections to provide additional editor annotations: code coverage, spelling
mistake... This was possible for C++ editor in previous version, using
CppModelManagerInterface::setExtraDiagnostics(), but this API has been removed.

This commits adds alternative API directly in the editor, allowing to pass a Core::Id instead of
the enum value.

Change-Id: I3040bd144d6fe0876d861dd28e832729bd5d6602
Reviewed-by: hjk <hjk121@nokiamail.com>