ProjectExplorer: Flat desktop device icon for flat themes

This change flattens the desktop device icon in the mode bar. Only for
themes which set the FlatSideBarIcons flag.

":///DESKTOP///" deduplicates to Constants::DESKTOP_DEVICE_ICON

Change-Id: Ie31d9c501db2a1e36edbfac59abdd22a4d8c6773
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/coreplugin/fancyactionbar.cpp

@@ -253,7 +253,7 @@ void FancyToolButton::paintEvent(QPaintEvent *event)
         }
 
         // pop up arrow next to icon
-        if (!icon().isNull()) {
+        if (isEnabled() && !icon().isNull()) {
             QStyleOption opt;
             opt.initFrom(this);
             opt.rect = rect().adjusted(rect().width() - 16, 0, -8, 0);