add openState to AbstractImporter to allow setting directly the state

src/Magnum/Trade/AbstractImporter.cpp

@@ -461,4 +461,8 @@ const void* AbstractImporter::importerState() const {
 
 const void* AbstractImporter::doImporterState() const { return nullptr; }
 
+void AbstractImporter::openState(const void* state) { return doOpenState(state); }
+
+void AbstractImporter::doOpenState(const void*) {}
+
 }}

src/Magnum/Trade/AbstractImporter.h

@@ -495,6 +495,8 @@ class MAGNUM_EXPORT AbstractImporter: public PluginManager::AbstractManagingPlug
          */
         const void* importerState() const;
 
+        void openState(const void* state);
+
     protected:
         /**
          * @brief Implementation for @ref openFile()
@@ -820,6 +822,9 @@ class MAGNUM_EXPORT AbstractImporter: public PluginManager::AbstractManagingPlug
 
         /** @brief Implementation for @ref importerState() */
         virtual const void* doImporterState() const;
+
+        /** @brief Implementation for @ref openState() */
+        virtual void doOpenState(const void* state);
 };
 
 CORRADE_ENUMSET_OPERATORS(AbstractImporter::Features)