Use bottom bar for previous/next navigation

app/src/main/java/eu/faircode/email/ViewModelMessages.java

@@ -0,0 +1,36 @@
+package eu.faircode.email;
+
+import androidx.lifecycle.ViewModel;
+import androidx.paging.PagedList;
+
+public class ViewModelMessages extends ViewModel {
+    private PagedList<TupleMessageEx> messages = null;
+
+    void setMessages(PagedList<TupleMessageEx> messages) {
+        this.messages = messages;
+    }
+
+    String[] getPrevNext(String thread) {
+        if (messages == null)
+            return new String[]{null, null};
+
+        boolean found = false;
+        TupleMessageEx prev = null;
+        TupleMessageEx next = null;
+        for (int i = 0; i < messages.size(); i++) {
+            TupleMessageEx item = messages.get(i);
+            if (item == null)
+                continue;
+            if (found) {
+                next = item;
+                messages.loadAround(i);
+                break;
+            }
+            if (thread.equals(item.thread))
+                found = true;
+            else
+                prev = item;
+        }
+        return new String[]{prev == null ? null : prev.thread, next == null ? null : next.thread};
+    }
+}