Read receipt support

Fixes #93

lib/helpers/extensions/is_shown_as_message.dart

@@ -0,0 +1,15 @@
+import "package:nexus/models/content/encrypted.dart";
+import "package:nexus/models/content/message.dart";
+import "package:nexus/models/content/sticker.dart";
+import "package:nexus/models/event.dart";
+
+extension IsShownAsMessage on Event {
+  /// Whether this is shown in the timeline as a message, with MessageRenderer.
+  bool get isShownAsMessage =>
+      redactedBy == null &&
+      relationType != "m.replace" &&
+      switch (content) {
+        MessageContent() || EncryptedContent() || StickerContent() => true,
+        _ => false,
+      };
+}