Grid: Add keyboard shortcuts for moving manual grid items

[noisysocks]

When a Grid block is set to Manual mode you can move the item around the grid using these custom movers in the toolbar:

You can also use drag-and-drop.

Let's add keyboard shortcuts for this as well.

Ideally I'd like:

• CmdOpt← - move left
• CmdOpt→ - move right
• CmdOpt↑ - move up
• CmdOpt↓ - move down

But I know sometimes existing browser keyboard shortcuts can get in the way of us having nice things.

Would be nice to have this for Move up and Move down in non-grid blocks too. (Maybe we already do?)