Create tel: links for phone numbers

Summary:
KDE Connect can handle tel: URLs and trigger a call, so this enables to
directly call phone numbers found e.g. in emails.

This is obviously a heuristic and might need more fine-tuning, as we can't
depend on libphonenumber here for more precision.

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D18369