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

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

Test Plan:
Before:
{F5681825}

After:
{F5681826}

Reviewers: #vdg, #kwin, fredrik

Reviewed By: #vdg, fredrik

Subscribers: ngraham, kwin, #kwin

Tags: #kwin

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