Qt patch backport request

[Martin Gräßlin]

Hi distributions,

Qt finally has a patch for the most often reported KWin issue. Please 
consider to backport: 
https://code.qt.io/cgit/qt/qtdeclarative.git/commit/?id=4d375f3f2b95bfcb322402df3525db6cc7a723ce

The bug in question on our side is: 
https://bugs.kde.org/show_bug.cgi?id=361236

Including the patch significantly improves the stability for our users 
if they use custom window decoration themes.

Cheers
Martin