[EuiIcon] Add icons to toggle panels visibility

[andreadelrio]

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

For the upcoming redesign of Discover, we are adding four new icons. This icon will be hideLeft, showLeft, hideTop, and showTop.

Describe the solution you'd like

Four new icons are added to EuiIcon.

Describe alternatives you've considered

We currently use eyeClosed and menuLeft and we believe these new custom icons will allow for a more consistent icon usage in Discover (and subsequently the rest of our apps).

Desired timeline

We would like to be able to use this before the 8.12 release