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