feat(Storybook): Utility proxy model for limiting model's count

It's convenient for testing some corner cases (especially when
used with Slider)  when e.g. model is empty or has only one item.

storybook/src/Storybook/LimitProxyModel.qml

@@ -0,0 +1,12 @@
+import SortFilterProxyModel 0.2
+
+SortFilterProxyModel {
+    id: root
+
+    property int limit
+
+    filters: IndexFilter {
+        maximumIndex: Math.max(root.limit - 1, 0)
+        minimumIndex: root.limit === 0 ? 1 : 0
+    }
+}

storybook/src/Storybook/qmldir

@@ -15,6 +15,7 @@ InspectionItem 1.0 InspectionItem.qml
 InspectionItemsList 1.0 InspectionItemsList.qml
 InspectionPanel 1.0 InspectionPanel.qml
 InspectionWindow 1.0 InspectionWindow.qml
+LimitProxyModel 1.0 LimitProxyModel.qml
 Logs 1.0 Logs.qml
 LogsAndControlsPanel 1.0 LogsAndControlsPanel.qml
 LogsView 1.0 LogsView.qml