Allow downloading keys from key backup

CMakeLists.txt

io.github.NhekoReborn.Nheko.yaml

resources/qml/EncryptionIndicator.qml

src/Cache.cpp

src/CacheCryptoStructs.h

src/Cache_p.h

src/ChatPage.cpp

src/ChatPage.h

src/MainWindow.cpp

src/Olm.cpp

src/Olm.h

src/UserSettingsPage.cpp

src/UserSettingsPage.h

src/timeline/EventStore.cpp

src/timeline/TimelineModel.cpp