[EuiBasicTable] Improve actions layout and interaction

[ryankeairns]

Is your feature request related to a problem? Please describe.
This request has two parts, the first being rather subjective design feedback, the second a request to make tables less 'noisy', visually.

Describe the solution you'd like

1. Decrease the gap between action buttons from 8 to 4.
2. Provide an option to 'show on hover' at the parent level - EuiTableActionsColumnType - even when there are less than three actions.

Describe alternatives you've considered

• Set the showOnHover for each action button

Desired timeline
Not urgent. The case that came up - dashboard listing page - is likely to be worked around by adding a third action.

Additional context
In having this discussion regarding the dashboard page, it felt like the 'show on hover' effect should work be generally available. Taken a step further, it could be the default behavior for all action columns.