Summary:
Currently kdevelop shows an "unknown" filetype in the Projects
manager, not perfect.

Given we already install a git icon, a simple fix is to also install some
mimetype definition. Ideally should be somewhere upstream, but until it has
been found out where and how, let's start for oursselved.

Reviewers: #kdevelop, apol

Reviewed By: #kdevelop, apol

Subscribers: apol, ltoscano, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D8746