Fixed possible duplication of Automapping Rules Tileset

* Since path normalization was changing "://automap-tiles.tsx" to
  ":/automap-tiles.tsx", copy/pasting tiles in a rule map could lead to
  duplication of this tileset. Fixed by adding the tileset using the
  normalized path.

* The action to add the Automapping Rules Tileset would always add
  another copy of the tileset, regardless of whether it was already
  present. This was a bug caused by using Tiled::readTileset instead of
  TilesetManager::loadTileset.

Closes #3462

NEWS.md

@@ -4,6 +4,7 @@
 * Added file system actions to the tile context menu (#3448)
 * Fixed possible crash in Custom Types Editor (#3465)
 * Fixed ability to reset nested string and file properties (#3409)
+* Fixed possible duplication of Automapping Rules Tileset (#3462)
 * Fixed case where object labels could become visible for hidden layer (#3442)
 * AutoMapping: Fixed applying of rule probability (#3425)
 

src/tiled/mainwindow.cpp

@@ -1868,7 +1868,7 @@ void MainWindow::addAutomappingRulesTileset()
     if (!mapDocument)
         return;
 
-    auto tileset = Tiled::readTileset(QStringLiteral("://automap-tiles.tsx"));
+    auto tileset = TilesetManager::instance()->loadTileset(QStringLiteral(":/automap-tiles.tsx"));
     if (!tileset)   // Should never happen, but better do nothing than crash
         return;