Updated blur default value from 5/15 to 10/15

Summary: The default blur value of 5/15 is too low.

Test Plan:
Before:

After:

Reviewers: VDG, KWin, fredrik

Reviewed By: VDG, fredrik

Subscribers: ngraham, kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D10180