Add KDEClangFormat cmake support

Summary:
This allows devs to run: "make clang-format" and format all files easily
using the preset KDE clang format style

This patch adds support so devs can easily test the intended formatting
now, it doesn't change the code yet..

If this patch is accepted I'll treat it as being accepted for all Plasma
repos. Doing an initial run to commit everything will happen later.

Test Plan:
Ran script
git diff was full of amazingly cleaned code

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25134