D25594: [Style KCM] Force active palette on widgets

[Kai Uwe Broulik]

broulik created this revision.
broulik added a reviewer: Plasma.
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REVISION SUMMARY
  Since there isn't a real window, the widgets think they're inactive, leading to dull appearance.
  Also, while at it create the `QPalette` only once. `KColorScheme::applicationPalette` parses configuration and is super slow. Doing it only once significantly speeds up the KCM.

TEST PLAN
  Before
  F7788650: Screenshot_20191128_161303.png <https://phabricator.kde.org/F7788650>
  After
  F7788649: Screenshot_20191128_161218.png <https://phabricator.kde.org/F7788649>

REPOSITORY
  R119 Plasma Desktop

REVISION DETAIL
  https://phabricator.kde.org/D25594

AFFECTED FILES
  kcms/style/previewitem.cpp

To: broulik, #plasma
Cc: plasma-devel, LeGast00n, The-Feren-OS-Dev, jraleigh, fbampaloukas, GB_2, ragreen, ZrenBot, ngraham, alexeymin, himcesjf, lesliezhai, ali-mohamed, jensreuterberg, abetts, sebas, apol, ahiemstra, mart
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