Minor UI improvements for the URLPopover

[afercia]

Description

Splitting this out from #58505

In the future, the URLPopover is going to be replaces wth the LinkControl UI, see #50893. In hte meantime, there's soom for some minor styling improvements though.

The main points reported in the feedback on #58505 are:

• Improve the layout of the link settings panel: uneven, inconsistent padding of the popover container, vertical spacing between input fields.
• Submit button label 'Apply' vs. 'Submit'.
• Consider to use the settings icon instead of the chevron down one.

For the relevant discussion and screenshots please see #58505

Step-by-step reproduction instructions

• See feedback on Restore default border and focus style on image url input field. #58505
• Observe the reported styling inconsistencies.

Screenshots, screen recording, code snippet

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Environment info

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Please confirm that you have searched existing issues in the repo.

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Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes