Mark classes as final where possible

src/AliasEditModel.h

src/AvatarProvider.h

src/BlurhashProvider.h

src/Cache_p.h

src/ChatPage.h

src/Clipboard.h

src/ColorImageProvider.h

src/CombinedImagePackModel.h

src/CompletionProxyModel.h

src/ImagePackListModel.h

src/InviteesModel.h

src/JdenticonProvider.h

src/MainWindow.h

src/MemberList.h

src/MxcImageProvider.h

src/PowerlevelsEditModels.h

src/ReadReceiptsModel.h

src/RoomsModel.h

src/SSOHandler.h

src/SingleImagePackModel.h