Check for dragability in TabularEditor should not construct drag item

Currently with the qt4 toolkit, the check for whether an item can be dragged actually calls `get_drag` and checks if the result is None:

https://github.com/enthought/traitsui/blob/dad91d3079725c4495f760884a463d2750a40bff/traitsui/qt4/tabular_model.py#L157-L161

This can lead to inefficiencies when constructing the drag item is expensive.