IMedia: Make thumbnail() const

include/medialibrary/IMedia.h

@@ -129,7 +129,7 @@ class IMedia
         /// \return A path, relative to the thumbnailPath configured when initializing
         ///  The media library
         ///
-        virtual const std::string& thumbnail() = 0;
+        virtual const std::string& thumbnail() const = 0;
         ///
         /// \brief isThumbnailGenerated Returns true if a thumbnail generation was
         ///                             attempted.

src/Media.cpp

@@ -267,7 +267,7 @@ Query<IAudioTrack> Media::audioTracks()
     return make_query<AudioTrack, IAudioTrack>( m_ml, "*", req, m_id );
 }
 
-const std::string& Media::thumbnail()
+const std::string& Media::thumbnail() const
 {
     if ( m_thumbnailId == 0 || m_thumbnailGenerated == false )
         return Thumbnail::EmptyMrl;

src/Media.h

@@ -124,7 +124,7 @@ class Media : public IMedia, public DatabaseHelpers<Media, policy::MediaTable>
         bool addAudioTrack( const std::string& codec, unsigned int bitrate, unsigned int sampleRate,
                             unsigned int nbChannels, const std::string& language, const std::string& desc );
         virtual Query<IAudioTrack> audioTracks() override;
-        virtual const std::string& thumbnail() override;
+        virtual const std::string& thumbnail() const override;
         virtual bool isThumbnailGenerated() const override;
         virtual bool setThumbnail( const std::string &thumbnail ) override;
         bool setThumbnail( const std::string& thumbnail, Thumbnail::Origin origin );