Summary: Makes for more idiomatic code. Bumps the required C++ standard to 14 to allow auto in lambdas. Requiring 14 should be fine, KWin alrady requires it.

Reviewers: #plasma, jgrulich

Reviewed By: jgrulich

Subscribers: davidre, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28696