configmgr: support writing back to dconf (WIP)

Needs to be enabled manually for now by changing CONFIGURATION_LAYERS's tail
from " user:..." to " user:*... dconf:!".

Change-Id: I31cd806f21d2ded376832faf98f49167b7243d1c

configmgr/source/components.cxx

@@ -291,7 +291,9 @@ void Components::writeModifications() {
             }
             break;
         case ModificationTarget::Dconf:
-            //TODO
+#if ENABLE_DCONF
+            dconf::writeModifications(*this, data_);
+#endif
             break;
         }
     }

configmgr/source/dconf.hxx

@@ -12,12 +12,17 @@
 
 #include <sal/config.h>
 
-namespace configmgr { struct Data; }
+namespace configmgr {
+    class Components;
+    struct Data;
+}
 
 namespace configmgr { namespace dconf {
 
 void readLayer(Data & data, int layer);
 
+void writeModifications(Components & components, Data & data);
+
 } }
 
 #endif

configmgr/source/modifications.hxx

@@ -45,6 +45,8 @@ public:
 
     void remove(Path const & path);
 
+    void clear() { root_.children.clear(); }
+
     bool empty() const { return root_.children.empty(); }
 
     Node const & getRoot() const { return root_;}