D27114: ScreenEdge: Do not use localtime for measuring duration

[Arjen Hiemstra]

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REVISION SUMMARY
  QDateTime::fromMSecSinceEpoch uses Qt::LocalTime by default. This involves an
  expensive localtime conversion. So instead force things to use UTC, as there
  is no need for timezone information when tracking durations.
  
  This is especially noticeable on Bedrock Linux, which uses a Fuse mounted
  /etc, which is slower than a plain /etc and causes quite some slowdown there.
  See https://github.com/bedrocklinux/bedrocklinux-userland/issues/140 for
  details.

TEST PLAN
  The screenedge unit test still passes.

REPOSITORY
  R108 KWin

BRANCH
  dont-use-localtime

REVISION DETAIL
  https://phabricator.kde.org/D27114

AFFECTED FILES
  plugins/platforms/x11/standalone/screenedges_filter.cpp
  screenedge.cpp
  tabbox/x11_filter.cpp

To: ahiemstra, #kwin
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