Fix QGIS plugin now that database entry is not in TOML (#826)

This code gives an error in the QGIS plugin:

```python
    def _get_database_path_from_model_file(self) -> str:
        with open(self.path, "rb") as f:
            model_filename = tomllib.load(f)["database"]
            return str(Path(self.path).parent.joinpath(model_filename))
```

Because the database entry is no longer present in the TOML because of
#815.

ribasim_qgis/widgets/dataset_widget.py

@@ -260,8 +260,10 @@ def load_geopackage(self) -> None:
 
     def _get_database_path_from_model_file(self) -> str:
         with open(self.path, "rb") as f:
-            model_filename = tomllib.load(f)["database"]
-            return str(Path(self.path).parent.joinpath(model_filename))
+            input_dir = Path(tomllib.load(f)["input_dir"])
+        # The .joinpath method (/) of pathlib.Path will take care of an absolute input_dir.
+        # No need to check it ourselves!
+        return str((Path(self.path).parent / input_dir / "database.gpkg").resolve())
 
     def new_model(self) -> None:
         """Create a new Ribasim model file, and set it as the active dataset."""