Mark own read messages with a double checkmark (#377)

resources/icons/ui/double-tick-indicator.png

resources/icons/ui/double-tick-indicator@2x.png

resources/res.qrc

src/Cache.cpp

src/Cache.h

src/ChatPage.cpp

src/timeline/TimelineItem.cpp

src/timeline/TimelineItem.h

src/timeline/TimelineView.cpp

src/timeline/TimelineView.h

src/timeline/TimelineViewManager.cpp

src/timeline/TimelineViewManager.h