[Yakuake] add blur effect support

Summary:
add an option to yakuakerc:

[Appearance]
Blur=true

see also:
https://www.reddit.com/r/kde/comments/6jzuwu/konsole_and_yakuake_blur/
https://www.reddit.com/r/kde/comments/c19yfj/simple_code_fix_yakuakes_blur_support/

Reviewers: Yakuake, hein

Reviewed By: hein

Subscribers: GB_2, ngraham, Yakuake

Tags: Yakuake

Differential Revision: https://phabricator.kde.org/D21867