[GTK3] CSD windows show handles on .solid-csd

Summary:
CSD windows will now show 5px handles on windows with .solid-csd (CSD windows in window managers that do not support GTK frame extents)

BUG: 412080
FIXED-IN: 5.17.0

Test Plan:
Handle before (KWin):
{F7396999}

Handle after (KWin):
{F7397796}

Reviewers: #breeze, ngraham, ndavis

Reviewed By: #breeze, ngraham, ndavis

Subscribers: ngraham, ndavis, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24126