Manage FAB more enabled state

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

@@ -1982,6 +1982,16 @@ public class FragmentMessages extends FragmentBase implements SharedPreferences.
         args.putLongArray("ids", getSelection());
 
         new SimpleTask<MoreResult>() {
+            @Override
+            protected void onPreExecute(Bundle args) {
+                fabMore.setEnabled(false);
+            }
+
+            @Override
+            protected void onPostExecute(Bundle args) {
+                fabMore.setEnabled(true);
+            }
+
             @Override
             protected MoreResult onExecute(Context context, Bundle args) {
                 long[] ids = args.getLongArray("ids");