Add Conan files

Summary:
Conan is a C++ package manager. This is an experiment to make KF5
frameworks available on Conan to make building KDE applications easier.
See also https://conan.io/ .

TODO:

• detect version number automatically

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: dfaure, apol, cfeck, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D6877