Improve time handling in NeoChat

This introduces a new NeoChatDateTime object that wraps a QDateTime. The intent is that it can be passed to QML and has a series of functions that format the QDateTime into the various string representations we need.

This means we only have to send the single object to QML and then the correct string can be grabbed from there, simplifying the backend. It is also easy to add a new representation if needed as a function with a QString output and Q_PROPERTY can be added and then it will be available.

src/timeline/models/messagemodel.h

@@ -60,8 +60,7 @@ public:
     enum EventRoles {
         DelegateTypeRole = Qt::UserRole + 1, /**< The delegate type of the message. */
         EventIdRole, /**< The matrix event ID of the event. */
-        TimeRole, /**< The timestamp for when the event was sent (as a QDateTime). */
-        SectionRole, /**< The date of the event as a string. */
+        DateTimeRole, /**< The timestamp for when the event was sent (as a NeoChatDateTime). */
         AuthorRole, /**< The author of the event. */
         HighlightRole, /**< Whether the event should be highlighted. */
         SpecialMarksRole, /**< Whether the event is hidden or not. */