callview: implement an observable object to detail the current call

CMakeLists.txt

extras/packaging/update/sparkle/Sparkle

src/app/calladapter.cpp

src/app/calladapter.h

src/app/currentcall.cpp

src/app/currentcall.h

src/app/mainview/MainView.qml

src/app/mainview/components/CallActionBar.qml

src/app/mainview/components/CallButtonDelegate.qml

src/app/mainview/components/CallOverlay.qml

src/app/mainview/components/CallStackView.qml

src/app/mainview/components/CallViewContextMenu.qml

src/app/mainview/components/InitialCallPage.qml

src/app/mainview/components/MainOverlay.qml

src/app/mainview/components/OngoingCallPage.qml

src/app/mainview/components/PluginHandlerPicker.qml

src/app/qmlregister.cpp

src/libclient/api/call.h

src/libclient/api/callmodel.h

src/libclient/callbackshandler.cpp