contacts: Add way to copy phone number easily

Yuki Joourequested to merge
yukijoou/merkuro:work/contact-copy-phone-number into master

Currently, clicking a phone number does nothing but logging an error about an undefined variable. Instead of this, this commit makes it copy the phone number.

This also replaces the generic arrow with a corresponding "copy" icon.

NOTE: I'm not a fan of the icon setting code being so verbose, but I don't think Kirigami has any easier APIs for it currently, unfortunately :c

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