gui: ImagesTreeView: rowsAboutToBeRemoved(): access proper index directly from node.

_parent may be invalid (for top nodes). Asking it for child will silently fail
gui: ImagesView: Data: assert added.
gui: DBDataModel: IdxData: removeChild() removed child (deletes memory). For ownership release takeChild() was introduced
gui: DBDataModel: IdxDataManager: Mode operation should not delete child item
gui: DBDataModel: IdxDataManager::photoChanged(): dealing with situation when 'idx' is invalid after findIdxDataFor()
Merge branch 'development' - fix for issue #82