Display block boundaries when cursor moves while typing

[aduth]

Closes #657

This pull request seeks to redisplay block boundaries if the user is currently typing within a block and then moves their cursor within said block. This is the final remaining task noted in #657.

Testing instruction:

1. Navigate to Gutenberg Demo editor
2. Select a block using mouse
3. Begin typing
4. Note that boundaries fade
5. Move cursor
6. Note that block boundaries reappear

[youknowriad]

What if we start moving the mouse elsewhere (outside the block boundaries), should we stop typing too?

[aduth]

@youknowriad I could see both arguments:

• That moving the mouse over the block is similar to the "hover" highlight, except the block's already selected so we show all controls.
• That moving the mouse is an intent that "you're done typing for now" regardless of where the cursor is

I'm more leaning toward the second of these two options.

cc @jasmussen

[jasmussen]

I'm leaning to the latter also.

[aduth]

Rebased to resolve conflicts and pushed 0b069fd0c0c14fa088e5164f11263c0b4ced455f which binds mousemove to entire document while block is being typed within, per discussion.

The block.js component is becoming increasingly unwieldy but I chose to add it here because some amount of typing behavior is necessary anyways (e.g. whether to show controls). Other consideration I had was a <BlockTypingTracker /> component to wrap <BlockEdit /> rendering and handle the events, or perhaps as a better fit for decorator type behavior proposed in #1023.

[youknowriad]

Maybe we could avoid this function

[aduth]

You mean binding this.props.onStopTying as the event handler? It's a little concerning to me since the onStopTyping prop may not be the same in the constructor as it is when the component later unmounts (or the event is bound), so we risk zombie event listeners. I'm actually unsure of the behavior of react-redux's mapDispatchToProps. Seems like it should only need to generate the props once when initializing, so it's likely not a concern in reality, just in theory.

[youknowriad]

LGTM 👍