TabularEditor issues

Issues found by going through TabularEditor demos in `examples/demos/Advanced` on MacOS system.

- [ ] 1. Column width is different between wx and qt implementations as seen in `Advanced/Tabular_editor_with_context_menu_demo.py`. I'm not sure which one is the correct one.

- [ ] 2. Wx has the feature to move rows using keyboard arrow buttons. The behaviour of the feature is inconsistent - with the first press of the button, the row moves down, however, selection stays on the same row index (essentially changes to the row that was above); second press moves the row down and also changes the selection so that the same row is selected. I might not be explaining very well, but the behaviour can be tested using `Advanced/Tabular_editor_demo.py` demo with wx backend.

- [ ] 3. Editing of certain values throws `TraitError`s. I have seen this with `Advanced/Tabular_editor_demo.py` "Spouse" column (read only) and `Advanced/Tabular_editor_with_context_menu_demo.py` "Ave" column (float). With PyQt5 this is a fatal error.

- [ ] 4. Insert functionality described in the documentation is broken in wx and likely not implemented in qt

Functionality differences:
- In Wx implementation left and right arrows move the rows. This doesn't work with qt

- In Qt implementation drag and drop allows moving of the rows. This doesn't work with wx

- In Wx implementation context menus don't work