Attach QP to body but position at top of main panel

[colin-grant-work]

What it does

On master, the Quick Pick can be occluded by the right side panel. This is a consequence of a recent change to attach the QP container to the ApplicationShell's main panel. This PR adjusts the logic for positioning the QP container to ensure that appears at the top of the main panel, but resides in the body.

How to test

1. Open a panel in the right panel and expand the panel.
2. Open a Quick Pick (e.g. using ctrl+p for file quick pick)
3. Observe that the QP appears at the top of the main panel (below the menu) and is not occluded by the right pane.
4. Enter Zen mode ('View: Toggle Maximized' with an editor visible) or reveal the toolbar ('View: Toggle Toolbar')
5. Repeat steps 2-3.

master:

This PR:

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

I confirm that the changes work as expected 👍

• the quick-input menu displays properly with different layouts
• the quick-input is correctly displayed above all other content (bug that the pr resolves)
• the quick-input works well with other content such as the toolbar (since it uses the main-panel)