Hidden events: Refactor slightly for reusability

CMakeLists.txt

io.github.NhekoReborn.Nheko.yaml

resources/qml/dialogs/HiddenEventsDialog.qml

resources/qml/dialogs/RoomSettings.qml

src/Cache.cpp

src/MainWindow.cpp

src/timeline/TimelineModel.cpp

src/timeline/TimelineModel.h

src/ui/HiddenEvents.cpp

src/ui/HiddenEvents.h

src/ui/RoomSettings.cpp

src/ui/RoomSettings.h