D27701: [ComboBox] Use transparent dimmer

[Kai Uwe Broulik]

broulik created this revision.
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REVISION SUMMARY
  When not using `dim` it breaks "click outside" for `ComboBox` when in a `ScrollView`.
  I believe because without an actual `Item` the `Popup` filters some events and might have them stolen by the `Flickable`?
  So we keep `dim` on but use a transparent `Item` to avoid it actually dimming its surroundings.

TEST PLAN
  - Kirigami gallery `ComboBox` still doesn't dim (it does without the `Item`)
  - `ComboBox` in QtQuick KCMs like KScreen KCM now close properly on click outside

REPOSITORY
  R858 Qt Quick Controls 2: Desktop Style

REVISION DETAIL
  https://phabricator.kde.org/D27701

AFFECTED FILES
  org.kde.desktop/ComboBox.qml

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