FeatureSet: Add some helper methods

I used to reimplement these a couple of times already.

Change-Id: I265cf50bf976638b4ac5406ae4010cf33c20354c
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/coreplugin/featureprovider.h

@@ -77,6 +77,11 @@ public:
        return *this;
     }
 
+    bool isEmpty() const
+    {
+        return QSet<Feature>::isEmpty();
+    }
+
     bool contains(const Feature &feature) const
     {
         return QSet<Feature>::contains(feature);
@@ -92,6 +97,11 @@ public:
         QSet<Feature>::remove(feature);
     }
 
+    void remove(const FeatureSet &features)
+    {
+        QSet<Feature>::subtract(features);
+    }
+
     FeatureSet operator|(const Feature &feature) const
     {
         FeatureSet copy = *this;
@@ -129,6 +139,14 @@ public:
             stringList.append(feature.toString());
         return stringList;
     }
+
+    static FeatureSet fromStringList(const QStringList &list)
+    {
+        FeatureSet features;
+        foreach (const QString &i, list)
+            features |= Feature(Id::fromString(i));
+        return features;
+    }
 };
 
 } // namespace Core