[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