Make roomlist look nice

resources/qml/RoomList.qml

src/CacheStructs.h

src/ChatPage.cpp

src/RoomList.cpp

src/timeline/RoomlistModel.cpp

src/timeline/RoomlistModel.h

src/timeline/TimelineModel.cpp

src/timeline/TimelineModel.h

src/ui/Theme.cpp

src/ui/Theme.h