Allow multiple of the old Properties Shelves to be open at once

The old Properties Shelf has been split into four separate shelves in Blender 2.8: Item, Tool, View, and Create. The problem is that you can only have one of these open at a time currently, and you have to physically click the tabs each time you want to open a different one.

This is much less efficient than the old singular Properties Shelf. There, you could expand/contract the various sections as needed, and then bring the whole shelf up with N. If you wanted to see the coordinates of a selected vertex and the coordinates of the 3D cursor for example, you could just see them both. Now you have to physically click between two different shelves, and you can only have one open at a time.

It's also not like there isn't plenty of space (especially with larger resolutions) to have more than one shelf open, here.

In Summary:

• Let people toggle the various shelves using the new buttons, Item, Tool, View, Create.
• Let people have more than one open at a time.
• Pressing N toggles all of the shelves you've enabled at once