D22531: [KColorSchemeManager] Optimize preview generation

[Kai Uwe Broulik]

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REVISION SUMMARY
  Create them only on-demand (e.g. when the menu opens or the DecorationRole is queried)
  and also use KConfig::SimpleConfig as we're passing in an absolute file path anyway.

TEST PLAN
  On Kate startup KColorSchemeManager went from 12% cost to 2.6% and saves around 40ms Kate startup time for me.
  Previews are still generated properly when I enter the menu. A `QComboBox` using the model still gets its
  previews generated when it reads the `DecorationRole`

REPOSITORY
  R265 KConfigWidgets

BRANCH
  ondemandpreview

REVISION DETAIL
  https://phabricator.kde.org/D22531

AFFECTED FILES
  src/kcolorschememanager.cpp
  src/kcolorschememanager_p.h

To: broulik, #frameworks
Cc: kde-frameworks-devel, LeGast00n, sbergeron, michaelh, ngraham, bruns
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