frontend: Use Qt parent-child ownership in OAuth dialog

[PatTheMav]

Description

Explicitly adds the OAuth dialog as a child element to its parent widget and removes the explicit delete call, as Qt will take care of cleanup as part of its ownership model.

Motivation and Context

This is a companion PR to obsproject/obs-browser#506. While it does not functionally require the change from obs-browser, it is only tested to work in tandem with it.

By explicitly setting the OAuth dialog as the parent of the CEF widget, it will automatically by cleaned up as soon as the dialog is closed itself.

This also detaches the destruction of the widget from the browser cleanup code.

Fixes #12706

How Has This Been Tested?

Tested on macOS and Windows by successfully establishing service connections and also aborting any such attempt to establish a connection after the login dialog had been opened.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.