D18655: Use correct window for getting ComboBox current icon pixmap

[Kai Uwe Broulik]

broulik created this revision.
broulik added reviewers: Plasma, hpereiradacosta.
Herald added a project: Plasma.
Herald added a subscriber: plasma-devel.
broulik requested review of this revision.

REVISION SUMMARY
  In widgets:
  
  - `windowHandle()` only works for top level widgets (windows), so we need to first get the actual `window()`, for controls like `ComboBox` this is always `nullptr`
  
  In QtQuick Controls:
  
  - we would just blow up for accessing `widget` which is null here. Use the `QQuickWindow` of the `QQuickItem`, if any

TEST PLAN
  5.15
  
  No longer blows up when I try to use icons in QQC2 desktop style ComboBox
  Didn't notice any visual difference in widget combobox rendering despite it now supposedly doing something wrt scaling?
  
  Compiled with and without `BREEZE_HAVE_QTQUICK`

REPOSITORY
  R31 Breeze

REVISION DETAIL
  https://phabricator.kde.org/D18655

AFFECTED FILES
  kstyle/breezestyle.cpp

To: broulik, #plasma, hpereiradacosta
Cc: plasma-devel, jraleigh, GB_2, ragreen, Pitel, ZrenBot, lesliezhai, ali-mohamed, jensreuterberg, abetts, sebas, apol, mart
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