Make it visually clear which ComboBox item is the current item

Do as the Default style does and make the current item bold. If we
don't do this, it can be impossible to distinguish the current item
from the highlighted item, especially when the popup obscures the
button.

Change-Id: If40b9c73c207d07fb5669564cdcfcea29ebed2f1
Fixes: QTBUG-68794
Reviewed-by: Nils Jeisecke <nils.jeisecke@saltation.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@qt.io>

src/imports/controls/fusion/ComboBox.qml

@@ -57,6 +57,7 @@ T.ComboBox {
     delegate: MenuItem {
         width: parent.width
         text: control.textRole ? (Array.isArray(control.model) ? modelData[control.textRole] : model[control.textRole]) : modelData
+        font.weight: control.currentIndex === index ? Font.DemiBold : Font.Normal
         highlighted: control.highlightedIndex === index
         hoverEnabled: control.hoverEnabled
     }

src/imports/controls/imagine/ComboBox.qml

@@ -61,6 +61,7 @@ T.ComboBox {
     delegate: ItemDelegate {
         width: parent.width
         text: control.textRole ? (Array.isArray(control.model) ? modelData[control.textRole] : model[control.textRole]) : modelData
+        font.weight: control.currentIndex === index ? Font.DemiBold : Font.Normal
         highlighted: control.highlightedIndex === index
         hoverEnabled: control.hoverEnabled
     }

src/imports/controls/universal/ComboBox.qml

@@ -58,6 +58,7 @@ T.ComboBox {
     delegate: ItemDelegate {
         width: parent.width
         text: control.textRole ? (Array.isArray(control.model) ? modelData[control.textRole] : model[control.textRole]) : modelData
+        font.weight: control.currentIndex === index ? Font.DemiBold : Font.Normal
         highlighted: control.highlightedIndex === index
         hoverEnabled: control.hoverEnabled
     }