Modularize documenation build

Modularize documenation build

qdocconf files can now reference $QT_INSTALL_DOCS to pick up e.g. global
includes, instead of using relative paths. Qt modules will automatically
get a doc target that builds and installs into the right place (including
supporting shadow-builds) if they set QMAKE_DOCS before loading(qt_module).

Change-Id: Ia408385199e56e3ead0afa45645a059d1a8b0d48
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>