[EuiComboBox] Clear button alignment

[acstll]

Is your feature request related to a problem? Please describe.

When the EuiComboBox has many selected values, the clear button can be hard to find because it's aligned center.

Describe the solution you'd like

Make the icon buttons top-aligned.

Additional context

Here's a codesandbox with the change with some custom CSS.

We need to consider on other inputs as well, not only the combobox.

[stratoula]

We have aligned the button at the top in these files elastic/kibana#217132

[weronikaolejniczak]

I'm reestimating this to 2. If the fix covered only the EuiComboBox, we could easily override and it'd be 1. If we are applying the fix globally, then we have to consider the impact, test different cases.