ongoingcallpage: local-preview: add a hide-preview feature

src/app/commoncomponents/LocalVideo.qml

src/app/commoncomponents/VideoView.qml

src/app/mainview/components/CallOverlay.qml

src/app/mainview/components/InitialCallPage.qml

src/app/mainview/components/OngoingCallPage.qml

src/app/net/jami/Constants/JamiStrings.qml

src/app/videodevices.cpp

src/libclient/api/avmodel.h

src/libclient/avmodel.cpp

src/libclient/qtwrapper/videomanager_wrap.h

tests/qml/main.cpp

tests/qml/src/TestWrapper.qml

tests/qml/src/tst_OngoingCallPage.qml