TempInfo Messageeventmodel

Further improvements to the handling of `mediaInfo` in `messageeventmodel`.

`blurhash` and `thumbnailInfo` are now replaced with a single `tempInfo` object that contains a single temp source. The priority is thumbnail > `blurhash` > empty if none exist.

Fixes network/neochat#235

src/qml/Component/Timeline/ImageDelegate.qml

@@ -74,7 +74,7 @@ TimelineContainer {
 
         Image {
             anchors.fill: parent
-            source: model.mediaInfo.blurhash
+            source: model.mediaInfo.tempInfo.source
             visible: parent.status !== Image.Ready
         }