Fix garbage collection of ListModelMap items

Prevent ListModel items from being deleted on the C++ side (specially
when using .get() from QML and a new ListModel is created)
by setting their parent to the ListModelMap.

harmonyqml/backend/model/list_model.py

@@ -5,15 +5,17 @@ from typing import (
 
 from namedlist import namedlist
 from PyQt5.QtCore import (
-    QAbstractListModel, QModelIndex, Qt, pyqtProperty, pyqtSlot
+    QAbstractListModel, QModelIndex, QObject, Qt, pyqtProperty, pyqtSlot
 )
 
 NewValue = Union[Mapping[str, Any], Sequence]
 
 
 class ListModel(QAbstractListModel):
-    def __init__(self, initial_data: Optional[List[NewValue]] = None) -> None:
-        super().__init__()
+    def __init__(self,
+                 initial_data: Optional[List[NewValue]] = None,
+                 parent:       Optional[QObject]        = None) -> None:
+        super().__init__(parent)
         self._ref_namedlist          = None
         self._roles: Tuple[str, ...] = ()
         self._list:  list            = []

harmonyqml/backend/model/list_model_map.py

@@ -8,7 +8,10 @@ from .list_model import ListModel
 class ListModelMap(QObject):
     def __init__(self) -> None:
         super().__init__()
-        self.dict: DefaultDict[Any, ListModel] = DefaultDict(ListModel)
+
+        # Set the parent to prevent item garbage-collection on the C++ side
+        self.dict: DefaultDict[Any, ListModel] = \
+            DefaultDict(lambda: ListModel(parent=self))
 
 
     @pyqtSlot(str, result="QVariant")
@@ -21,6 +24,7 @@ class ListModelMap(QObject):
 
 
     def __setitem__(self, key, value: ListModel) -> None:
+        value.setParent(self)
         self.dict[key] = value