Adhere to ipywidget standards in trait names.

As [proposed](https://github.com/aiidalab/aiidalab-widgets-base/pull/69#issuecomment-610366361) by @csadorf:

We should consider renaming the primary "value" of a widget, actually `value`, since the type is already specified through the class name. For example, the "value" of an `IntSlider` is not `number` or `integer_number`, but `value`.

In the case of computers it would look as follows:
- `selected_computer` -> `value`
- `computers` -> `options`

I think we should go through the deprecation mechanism here, to be decided.