CachedImage: add icon downloader for pluginstore and welcome page

CMakeLists.txt

src/app/MainApplicationWindow.qml

src/app/imagedownloader.cpp

src/app/imagedownloader.h

src/app/mainapplication.cpp

src/app/mainapplication.h

src/app/mainview/components/CachedImage.qml

src/app/qmlregister.cpp

src/app/qmlregister.h

src/app/utilsadapter.cpp

src/app/utilsadapter.h

src/app/webengine/GeneralWebEngineView.qml

tests/qml/main.cpp

tests/qml/src/tst_CachedImage.qml