Added multiple selection

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

@@ -0,0 +1,36 @@
+package eu.faircode.email;
+
+import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+import androidx.paging.PagedList;
+import androidx.recyclerview.selection.ItemKeyProvider;
+import androidx.recyclerview.widget.RecyclerView;
+
+public class ItemKeyProviderMessage extends ItemKeyProvider<Long> {
+    private RecyclerView recyclerView;
+
+    ItemKeyProviderMessage(RecyclerView recyclerView) {
+        super(ItemKeyProvider.SCOPE_CACHED);
+        this.recyclerView = recyclerView;
+    }
+
+    @Nullable
+    @Override
+    public Long getKey(int pos) {
+        AdapterMessage adapter = (AdapterMessage) recyclerView.getAdapter();
+        return adapter.getCurrentList().get(pos).id;
+    }
+
+    @Override
+    public int getPosition(@NonNull Long key) {
+        AdapterMessage adapter = (AdapterMessage) recyclerView.getAdapter();
+        PagedList<TupleMessageEx> messages = adapter.getCurrentList();
+        if (messages != null)
+            for (int i = 0; i < messages.size(); i++) {
+                TupleMessageEx message = messages.get(i);
+                if (message != null && message.id.equals(key))
+                    return i;
+            }
+        return RecyclerView.NO_POSITION;
+    }
+}