D8436: use a single QML engine

[Marco Martin]

mart created this revision.
mart added a reviewer: Plasma.
Restricted Application added projects: Plasma, Frameworks.
Restricted Application added subscribers: Frameworks, plasma-devel.

REVISION SUMMARY
  use a shared qml engine for all ConfigModules in the same process
  this avoids some crashes as creating and deleting stuff from
  different engines is known to cause crashes in the v4 garbage collection
  and is recomended to use a single QQmlEngine per process

TEST PLAN
  normal SystemSettings seems unaffected, plasma mobile systemsettings
  which is qml-only now doesn't crash anymore after loading a module
  for the second time

REPOSITORY
  R296 KDeclarative

BRANCH
  phab/singleengine

REVISION DETAIL
  https://phabricator.kde.org/D8436

AFFECTED FILES
  src/quickaddons/configmodule.cpp

To: mart, #plasma
Cc: plasma-devel, #frameworks, ZrenBot, progwolff, lesliezhai, ali-mohamed, jensreuterberg, abetts, sebas, apol, mart
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