Qt Help should use domain "qt-project.org" instead of "trolltech.com"

Currently, the domain "trolltech.com" is hardcoded in many places:

• various *.qdocconf files in module qtdoc
• assistant sources (link to internal docs)
• assistant autotests
• Designer code launching assistant in qttools/src/designer/src/designer/assistantclient.cpp

This should be replaced by "qt-project.org".

HOWTO:

• Do a prefix-build of Qt 5
• in qtdoc, run: make qch_docs ; cd doc ; make install_qchdocs
• Assistant should then find the Documentation