ui: export descriptor

[reez]

Description

Try's to solve #230

Still a WIP and not exactly what we want because we don't have these yet:

• Only copies descriptor, but not change descriptor
• does not have "share" button (other related request)
• descriptor format is prv
• descriptor format uses tick not "h" /86'/1'/0'/0/*

Notes to the reviewers

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I have formatted my code with swift-format per .swift-format file

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR