Reset user colors on theme change (in qml timeline)

resources/qml/TimelineRow.qml

resources/qml/TimelineView.qml

resources/qml/delegates/MessageDelegate.qml

resources/qml/delegates/Reply.qml

src/Cache.cpp

src/Cache.h

src/Cache_p.h

src/MainWindow.cpp

src/timeline/TimelineModel.cpp

src/timeline/TimelineModel.h

src/timeline/TimelineViewManager.cpp

src/timeline/TimelineViewManager.h