unreads: Add helpers to provide unread counts for narrows

These helpers do not account for muted topics and streams,
those are left for zulip#346.

Author: sirpengi

lib/model/unreads.dart

@@ -125,6 +125,50 @@ class Unreads extends ChangeNotifier {
 
   int countInDmNarrow(DmNarrow narrow) => dms[narrow]?.length ?? 0;
 
+  // TODO(#346): account for muted topics and streams
+  int countInAllMessagesNarrow() {
+    int c = 0;
+    for (final messageIds in dms.values) {
+      c = c + messageIds.length;
+    }
+    for (final topics in streams.values) {
+      for (final messageIds in topics.values) {
+        c = c + messageIds.length;
+      }
+    }
+    return c;
+  }
+
+  // TODO(#346): account for muted topics and streams
+  int countInStreamNarrow(StreamNarrow narrow) {
+    final topics = streams[narrow.streamId];
+    if (topics == null) return 0;
+    int c = 0;
+    for (final messageIds in topics.values) {
+      c = c + messageIds.length;
+    }
+    return c;
+  }
+
+  // TODO(#346): account for muted topics and streams
+  int countInTopicNarrow(TopicNarrow narrow) {
+    final topics = streams[narrow.streamId];
+    return topics?[narrow.topic]?.length ?? 0;
+  }
+
+  int countInNarrow(Narrow narrow) {
+    switch (narrow) {
+      case DmNarrow():
+        return countInDmNarrow(narrow);
+      case AllMessagesNarrow():
+        return countInAllMessagesNarrow();
+      case StreamNarrow():
+        return countInStreamNarrow(narrow);
+      case TopicNarrow():
+        return countInTopicNarrow(narrow);
+    }
+  }
+
   void handleMessageEvent(MessageEvent event) {
     final message = event.message;
     if (message.flags.contains(MessageFlag.read)) {