Clean up type selection combobox class hierarchy

Summary:
Having the URL one inherit the email one makes no sense, so decouple them,
remove the virtual method now no longer needed, and move the email one to
its sub-directory.

Prepares this for porting the email editor to the new KContacts API rather
than using string-based access.

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D21018