shortcut: add option to edit last sent message

src/app/currentconversation.cpp

src/app/currentconversation.h

src/app/mainview/components/ChatView.qml

src/app/mainview/components/KeyboardShortcutTable.qml

src/app/mainview/components/MessageBarTextArea.qml

src/libclient/api/conversation.h

src/libclient/conversationmodel.cpp

src/libclient/messagelistmodel.cpp

src/libclient/messagelistmodel.h