Add 'Send via email...' plugin

Summary:
Allows sending files as attachments via email. The email client is opened via
the mailto: schema using QDesktopService. The only exception is when Thunderbird
is configured as a default email client, then we invoke Thunberbird directly
with special commandline argument, since Thunderbird cannot handle attachments
via the mailto URL scheme.

Test Plan: Tested with KMail, Evolution and Thunderbird.

Reviewers: apol

Differential Revision: https://phabricator.kde.org/D8238