Allow opening message menus for out-of-room events

These are more common than we thought, good examples are pinned or
searched messages - which are not going to be in the room's history
unless you happen to have them loaded. But currently our message menu
infrastructure expects them to be, since its looked up by the room +
event ID.

To fix this is simple, we now move the job of finding the event to the
caller which may use a model instead. I didn't fix all existing
call-sites yet, mainly the message menu opening one since that was the
most obvious bug. But this opens up the door for other assumptions about
room history to be fixed too.

I had to do a bit of C++ re-jiggering in order to expose useful
functions to QML.

src/timeline/models/messagemodel.cpp

@@ -351,16 +351,21 @@ QHash<int, QByteArray> MessageModel::roleNames() const
 
 QModelIndex MessageModel::indexForEventId(const QString &eventId) const
 {
-    if (m_room == nullptr) {
-        return {};
-    }
-
-    const auto it = m_room->findInTimeline(eventId);
-    if (it == m_room->historyEdge()) {
+    const auto matches = match(index(0, 0), EventIdRole, eventId);
+    if (matches.isEmpty()) {
         qWarning() << "Trying to find non-existent event:" << eventId;
         return {};
     }
-    return index(it - m_room->messageEvents().rbegin() + timelineServerIndex());
+    return matches.constFirst();
+}
+
+const RoomEvent *MessageModel::findEvent(const QString &eventId) const
+{
+    const auto index = indexForEventId(eventId);
+    if (const auto event = getEventForIndex(index)) {
+        return &event.value().get();
+    }
+    return nullptr;
 }
 
 void MessageModel::fullEventRefresh(int row)