[partition] Extra helper classes for doing reset and refresh

- The ResetHelper only finalized changes to the module on
   destruction, but calls to refresh() assumed it was already
   done. This leads to crashes when refresh() uses an intermediate
   state of the model.

Introduce extra helpers, and rename refresh() to avoid calling the
old implementation from any code. The new helper just creates and
destroys a ResetHelper, before creating and destroying an object
that calls the new refreshAfterModelChange().

FIXES #1019

src/modules/partition/core/PartitionCoreModule.h

@@ -54,6 +54,25 @@ class PartitionCoreModule : public QObject
 {
     Q_OBJECT
 public:
+    /**
+     * This helper class calls refresh() on the module
+     * on destruction (nothing else). It is used as
+     * part of the model-consistency objects, along with
+     * PartitionModel::ResetHelper.
+     */
+    class RefreshHelper
+    {
+    public:
+        RefreshHelper( PartitionCoreModule* module );
+        ~RefreshHelper();
+
+        RefreshHelper( const RefreshHelper& ) = delete;
+        RefreshHelper& operator=( const RefreshHelper& ) = delete;
+
+    private:
+        PartitionCoreModule* m_module;
+    };
+
     /**
      * @brief The SummaryInfo struct is a wrapper for PartitionModel instances for
      * a given Device.
@@ -192,7 +211,7 @@ Q_SIGNALS:
     void deviceReverted( Device* device );
 
 private:
-    void refresh();
+    void refreshAfterModelChange();
 
     /**
      * Owns the Device, PartitionModel and the jobs