revamp doc generation targets

revamp doc generation targets

- the old docs target becomes html_docs
- a new qch_docs target is added. the .qch files end up directly in
  QT_INSTALL_DOCS, wihout any subdirectories in between
- the new docs target invokes html_docs and qch_docs
- respective un-/install targets are added as well. note that the
  install targets don't depend on the build targets, as it's virtually
  impossible to get the dependencies right throughout the hierarchy.

Change-Id: I07a2589db8252371e77cf925c47c4e59fbd1b2ca
Reviewed-by: hjk <qthjk@ovi.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>