Blocks: Improve block mover scroll position preserve

[aduth]

Related: #3546

The current behavior for preserving scroll offset when moving blocks checks whether the current block is selected:

gutenberg/editor/components/block-list/block.js

Lines 120 to 128 in c52e0b4

	componentWillReceiveProps( newProps ) {
	if (
	this.props.order !== newProps.order &&
	( ( this.props.isSelected && newProps.isSelected ) ||
	( this.props.isFirstMultiSelected && newProps.isFirstMultiSelected ) )
	) {
	this.previousOffset = this.node.getBoundingClientRect().top;
	}
	}

This means that when merely hovering a block and using the Block Mover arrows to rearrange the block, scroll will not be preserved on the first click. Subsequent clicks will preserve position because the block is automatically selected when moved.

We should consider...

• Refactoring to rely less on explicit selected state
• Ensure that isSelected takes effect before deciding to assign this.previousOffset