fix(window level): The window level value is not displayed by default on all the viewports when selecting common/custom layout and TMTV. (#5865)

* fix(window level): Set up listener for viewport availability such that the initial window level can be read and displayed.

* PR feedback.

* PR feedback.

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Co-authored-by: Bill Wallace <wayfarer3130@gmail.com>

tests/SegmentationPanel.spec.ts

@@ -44,7 +44,7 @@ test('checks saved segmentations loads and jumps to slices', async ({
   rightPanelPageObject,
   viewportPageObject,
 }) => {
-  const viewportInfoBottomRight = viewportPageObject.active.overlayText.bottomRight;
+  const viewportInfoBottomRight = viewportPageObject.active.overlayText.bottomRight.instanceNumber;
   // Image loads on slice 1, confirm on slice 1
   await expect(viewportInfoBottomRight).toContainText('1/', { timeout: 10000 });