Implement avatars in qml timeline

CMakeLists.txt

resources/qml/Avatar.qml

resources/qml/TimelineView.qml

resources/res.qrc

src/MxcImageProvider.cpp

src/MxcImageProvider.h

src/RoomInfoListItem.cpp

src/UserSettingsPage.cpp

src/timeline2/TimelineModel.cpp

src/timeline2/TimelineModel.h

src/timeline2/TimelineViewManager.cpp

src/ui/Avatar.cpp