Update ConversationMessage.qml to latest version used in Kaidan
Summary
Kaidan has added several nice features to their ChatMessage, including right-click to copy and nice Avatars! There are actually more features that I can't support without significant modification, which I will not do until I start abstracting that code into a library that both projects can share
TODO: Move all Kaidan GUI resources into a library shared by both projects. See: https://invent.kde.org/kde/kaidan/issues/310