Add an index on filename on songs table. Particullary, this heavily speed up the 'Add folder' usecase

data/data.qrc

@@ -334,5 +334,6 @@
         <file>clementine-spotify-public.pem</file>
         <file>icons/22x22/user-away.png</file>
         <file>icons/32x32/search.png</file>
+        <file>schema/schema-35.sql</file>
     </qresource>
 </RCC>

data/schema/schema-35.sql

@@ -0,0 +1,3 @@
+CREATE INDEX idx_filename ON songs (filename);
+
+UPDATE schema_version SET version=35;

src/core/database.cpp

@@ -33,7 +33,7 @@
 #include <QVariant>
 
 const char* Database::kDatabaseFilename = "clementine.db";
-const int Database::kSchemaVersion = 34;
+const int Database::kSchemaVersion = 35;
 const char* Database::kMagicAllSongsTables = "%allsongstables";
 
 int Database::sNextConnectionId = 1;