Add basic support for multiple windows

resources/qml/MessageView.qml

resources/qml/RoomList.qml

resources/qml/Root.qml

src/ChatPage.cpp

src/ChatPage.h

src/MainWindow.h

src/timeline/RoomlistModel.cpp

src/timeline/RoomlistModel.h

src/timeline/TimelineModel.cpp

src/timeline/TimelineViewManager.cpp

src/timeline/TimelineViewManager.h