[electrum] minor improvements to proof editor

Summary:

• Add a "copy to clipboard" button to Master key fields.
• Make it visually clear that the master pubkey is not editable (on some systems setReadOnly does not modify the style)
• Improve tooltips
• Improve labels

Test Plan: open the proof editor in any wallet

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14602