Need to focus in Sequencer window for arrow hotkeys to work each time when scrubbing the timeline

The Sequencer timeline can't be scrubbed with the arrow keys because the Sequencer Window loses focus after an arrow key is pressed. This is a regression from 4.21.2 (CL-4753647) where repeated arrow key presses will scrub the timeline continually.

This was reported and tested in 4.22 Preview 7 (CL-5531554). This was reproduced in Main 4.23 (CL-5552558)

1. Open the UE4 Editor and create a Blank template project
2. Create a new Level Sequence and click on the timeline
3. Press Right or Left on the arrow keys
4. Repeat Step 3

Results: Nothing happens

Expected: The timeline moves one frame to the left or right again