[partition] Tighten up types

Don't return the generic Abstract model for bootloader, but
the subclass pointer, so that consumers can use the convenience
API on the subclass.

src/modules/partition/core/PartitionCoreModule.cpp

@@ -340,7 +340,7 @@ PartitionCoreModule::deviceModel() const
     return m_deviceModel;
 }
 
-QAbstractItemModel*
+BootLoaderModel*
 PartitionCoreModule::bootLoaderModel() const
 {
     return m_bootLoaderModel;

src/modules/partition/core/PartitionCoreModule.h

@@ -122,7 +122,7 @@ public:
      * The single BootLoaderModel instance belongs to the PCM.
      * @return the BootLoaderModel.
      */
-    QAbstractItemModel* bootLoaderModel() const;
+    BootLoaderModel* bootLoaderModel() const;
 
     void createPartitionTable( Device* device, PartitionTable::TableType type );