Added multiple selection

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

@@ -0,0 +1,22 @@
+package eu.faircode.email;
+
+import androidx.annotation.NonNull;
+import androidx.recyclerview.selection.SelectionTracker;
+
+public class SelectionPredicateMessage extends SelectionTracker.SelectionPredicate<Long> {
+
+    @Override
+    public boolean canSetStateForKey(@NonNull Long key, boolean nextState) {
+        return true;
+    }
+
+    @Override
+    public boolean canSetStateAtPosition(int position, boolean nextState) {
+        return true;
+    }
+
+    @Override
+    public boolean canSelectMultiple() {
+        return true;
+    }
+}