Change project name to "KWin"

Summary: It's not "KWIN" nor "kWin" nor "kWIN" nor "kwin"

Test Plan: Qt Creator displays "KWin" with correct capitalization.

Reviewers: #kwin, meven

Reviewed By: meven

Subscribers: apol, meven, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D26500