Refactor sprite info behavior into a container and let it handle the raw events.

TODO: refactor again into redux

Very crude, requires a refactor to allow toggling

For the time being, update the position onChange. Eventually this will update onBlur/on "enter" key

Resolves #16

/ht @rschamp

Improves performance by comparing sprite info values (rather than object identities), and by excluding irrelevant data from this comparison.

`getBoundingClientRect` forces WebKit to recalculate the layout ("force reflow"), so cache the canvas bounding box rather than getting it on every mouse event.

Change Toolbox Categories to match new design color

The `info` property seems to refer to the image size, not the rotation center in the backdrop library. So if it exists, halve it to find the center.

Towards #18

Remove MediaLibrary helper since it is now unused

This will allow the GUIComponent to have control over the layout

The inline css method gives us nothing that CSS modules can't give us. This puts the project more in line with the way www works, and makes CSS actually readable.

Using class names gives us full control of the modal styles.

Refactor SpriteSelector into "Target Pane" which contains the sprite selector, stage selector, and the buttons for new sprites, costumes and backdrops.

Add a new stage selection component that displays the stage separately from the sprite list.

In preparation to split the sprites from the stage, start managing the target list state in a single place.

For now this uses the CSS `filter` property to crudely "highlight" for the active state. This is a temporary solution until we have a real design for the active states.

These were named inconsistently in scratch-vm

After doing this, new rules began applying for some reason:

`eol-last` only began taking the 'always' parameter as of 3.6.0, so I needed to update eslint.
`no-unused-var` started warning about process.env after updating the config structure
`comma-dangle` decided to start working, maybe because of the upgrade to eslint?  Autofixed with --fix