[effects/blur] Clean up shader code

Summary:
* Drop abstract BlurShader class
* Delete evil "using namespace KWin"
* Fix includes
* Use smart pointers
* Turn BlurShader into a QObject
* Fix coding style
* Add missing default cases
* Use default member initialization
* Delete methods that are used only once
* Use more const
* Use QRect::{top,right,bottom,left} methods in the setBlurRect method

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13110