Breadcrumb and visual improvements

[jeffreyvr]

• The editor now includes a breadcrumb, allowing you to easily switch to a parent block. You may also use the arrow up key.
• Blocks can be hidden from the block inserter (sidebar) by setting asChildOnly to true. Only when this block is then explicitly allowed, it will become visible in the block inserter.
• A block now has an isInEditor method to check if the block is currently being displayed in the editor. This will allow you to make the block look/behave slightly different in the editor.
• Several visual changes to more clearly indicate the currently active or hovered block, and showing the block name alongside the block toolbar.