Rerefences to code snippets that are moved or missing

A number of documentation pages still don't show snippets (they're shown as empty code blocks).

cpp snippets; one or more empty code blocks on these pages:

./qtbase/doc/activeqt/activeqt-multiple.html
./qtbase/doc/activeqt/activeqt-opengl.html
./qtbase/doc/activeqt/activeqt-simple.html
./qtbase/doc/activeqt/activeqt-wrapper.html
./qtbase/doc/qtcore/customcompleter.html
./qtbase/doc/qtcore/plugins-howto.html
./qtbase/doc/qtcore/qtplugin.html
./qtbase/doc/qtcore/resources.html
./qtbase/doc/qtdesigner/designer-creating-custom-widgets.html
./qtbase/doc/qtgui/coordsys.html
./qtbase/doc/qtgui/coordsys.html
./qtbase/doc/qtmultimedia/qaudiodeviceinfo.html
./qtbase/doc/qtmultimedia/qaudioinput.html
./qtbase/doc/qtmultimedia/qaudiooutput.html
./qtbase/doc/qtmultimedia/qcamerafocus.html
./qtbase/doc/qtmultimedia/qcamerafocuszone.html
./qtbase/doc/qtmultimedia/qcamera.html
./qtbase/doc/qtmultimedia/qcameraimagecapture.html
./qtbase/doc/qtmultimedia/qcameraimageprocessing.html
./qtbase/doc/qtmultimedia/qcameraviewfinder.html
./qtbase/doc/qtmultimedia/qgraphicsvideoitem.html
./qtbase/doc/qtmultimedia/qmediaplayer.html
./qtbase/doc/qtmultimedia/qmediaplaylist.html
./qtbase/doc/qtmultimedia/qsoundeffect.html
./qtbase/doc/qtmultimedia/qsound.html
./qtbase/doc/qtmultimedia/qvideorenderercontrol.html
./qtbase/doc/qtmultimedia/qvideowidgetcontrol.html
./qtbase/doc/qtmultimedia/qvideowidget.html
./qtbase/doc/qtmultimedia/qvideowindowcontrol.html
./qtbase/doc/qtquick/qml-qtquick-xmllistmodel2-xmlrole.html
./qtbase/doc/qtquick/qquickimageprovider.html
./qtbase/doc/qtwidgets/gestures-overview.html
./qtbase/doc/qtwidgets/qmaccocoaviewcontainer.html
./qtbase/doc/qtwidgets/qmacnativewidget.html
./qtbase/doc/qtxml/qxmldefaulthandler.html

qml snippets; one of more empty code blocks on these pages:

./qtbase/doc/qtmultimedia/qml-qtmultimedia5-soundeffect.html
./qtbase/doc/qtqml/quick-particles-imageparticle.html
./qtbase/doc/qtquick/qml-color.html
./qtbase/doc/qtquick/qml-qt-labs-folderlistmodel1-folderlistmodel.html
./qtbase/doc/qtquick/qquickimageprovider.html
./qtbase/doc/qtquick/quick-particles-imageparticle.html