Add menu to enable or disable stickers globally

CMakeLists.txt

resources/qml/RoomSettings.qml

resources/qml/Root.qml

resources/qml/dialogs/ImagePackSettingsDialog.qml

resources/res.qrc

src/Cache.cpp

src/CacheStructs.h

src/Cache_p.h

src/CombinedImagePackModel.cpp

src/ImagePackListModel.cpp

src/ImagePackListModel.h

src/SingleImagePackModel.cpp

src/SingleImagePackModel.h

src/timeline/TimelineViewManager.cpp

src/timeline/TimelineViewManager.h

src/ui/RoomSettings.h