Make BuildStepConfigWidget hideable

Some steps only want to show a summary text.

Change-Id: I125fa90c13f0757ea78b6b50834c43b1beb484c5
Reviewed-by: Nikolai Kosjar
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>

src/libs/utils/detailswidget.h

@@ -56,7 +56,8 @@ public:
     enum State {
         Expanded,
         Collapsed,
-        NoSummary
+        NoSummary,
+        OnlySummary
     };
 
     explicit DetailsWidget(QWidget *parent = 0);