Try to reduce memory usage by reusing avatar pixmaps

src/AvatarProvider.cpp

src/AvatarProvider.h

src/Cache.cpp

src/Cache.h

src/ChatPage.cpp

src/ChatPage.h

src/Logging.cpp

src/Logging.h

src/RoomInfoListItem.cpp

src/RoomInfoListItem.h

src/RoomList.cpp

src/RoomList.h

src/TopRoomBar.cpp

src/TopRoomBar.h

src/UserInfoWidget.cpp

src/UserInfoWidget.h

src/dialogs/MemberList.cpp

src/dialogs/ReadReceipts.cpp

src/dialogs/RoomSettings.cpp

src/dialogs/RoomSettings.h