TableEditor: Qt: Fix exception in selected_indices

The `_get_selected_indices` method was incorrectly implemented
causing TypeError.
TableModel: Qt: return valid flag from overridden Qt method.

Avoid warning messages of the kind:

 RuntimeWarning: Invalid return value in function
 QAbstractTableModel.flags, expected ItemFlags, got NoneType.
TableEditor: Qt: CheckBoxColumn: Draw correct inactive background
Merge pull request #167 from pankajp/fix-qt-table-editor-selection_indices

TableEditor: Qt: Fix exception in selected_indices and more