Icons: Use theme color for disabled icon state

The color role was already availabe in Utils::Theme, but
Utils::Icon didn't yet make use of it. That was a bug.

Change-Id: Ifb99e08567223c0999296fde34f61de21b764b14
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

share/qtcreator/themes/dark.creatortheme

@@ -6,7 +6,7 @@ DefaultTextEditorColorScheme=dark.xml
 [Palette]
 shadowBackground=ff232323
 text=ffe7e7e7
-textDisabled=ffa0a0a0
+textDisabled=ff909090
 hoverBackground=ff515151
 selectedBackground=ff151515
 normalBackground=ff333333

src/libs/utils/icon.cpp

@@ -184,8 +184,7 @@ QIcon Icon::icon() const
         const QPixmap combinedMask = Utils::combinedMask(masks, m_style);
         result.addPixmap(masksToIcon(masks, combinedMask, m_style));
 
-        QColor disabledColor = creatorTheme()->palette().mid().color();
-        disabledColor.setAlphaF(0.6);
+        const QColor disabledColor = creatorTheme()->color(Theme::IconsDisabledColor);
         result.addPixmap(maskToColorAndAlpha(combinedMask, disabledColor), QIcon::Disabled);
         return result;
     }