Reset visual item parent on item delete request

[maxhora]

The change should resolve UI glitches, for example, on array of react-native components (issues with messages list in status-react app).
Description of issue:

• react-native has optimization to render array of UI components. Implementation is done in function reconcileChildrenArray ( file Libraries/Renderer/ReactNativeRenderer-dev.js ). On recalculation old instances might be reused for new components instances.
• Sometimes arrays optimization deletes multiple UI components one by one too fast and QML scene is not able to recalculate child items for the parent container from where we resolve indexes of child items to be removed. (API doesn't exist to ask parent QML item directly to remove some its child)
• Resetting ParentItem on child to nullptr seems asks parent to remove child from children list immediately, that resolves the issue.