Inner Blocks - Floating Toolbar

[pinarol]

This is a kind of toolbar that hovers over the selected block.

Please take a look at the prototype to get a sense of how it should behave.

Note: This is only visible when the user has a parent block on the canvas (something with children in it).

For this particular issue let's just add a Floating Toolbar that has no functionality other than hovering. We can put a non functioning upwards button in it and ignore the breadcrumbs part.

We'll be putting the real functionality with issues:
Inner Blocks - Navigating Upward in the hierarchy
Inner Blocks - Breadcrumbs