* Covers parts of these issues: GUI layout panes (#69), sprite libraries (#12), import buttons (#64), menu bar (#67), and sprite info area (#53)

~ Refactored layout pane structure by removing `display: flex` from `.box`, by default. Declaring flex instead only where specifically needed. This helped fix layout quirks, and should help us avoid hard to find issues related to nested flex boxes

~ Added descriptive classnames to boxes/containers/wrappers. Moved more padding, margins + layout into these wrappers to help with component reusability.

~ Set up a spacer unit variable inside `gui.css`, to start keeping consistent padding, margins and rounded corners between panes. Used [CSS/PostCSS Modules](https://github.com/css-modules/postcss-modules-values) to avoid SCSS dependency.

~ Refactored inline styles from JSX, into CSS files. 2 reasons: a) prepping for reuse of CSS Module variables. b) Feels easier to debug complex flex layouts when all the styles are in one place vs JS/HTML/CSS syntax mixed together

~ Added menu bar into layout via new component, and sprite info header in sprite selector pane, with stubs for the labels + forms. Sprite items are correctly scrolling, while leaving the header fixed, in Chrome. Mostly working in FFx. Not working in Safari, IE yet.

~ Style pass on library modal

~ Experimenting with a few different transition styles: Logo in menu bar, sprite library items, Add buttons.

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Dynamic menus for costumes and backdrops

PygmyMarmosetPatch for sound menu updates on workspace update

Monkeypatch dynamic sound menu into ScratchBlocks

/ht @rschamp

Implement shouldComponentUpdate on TargetPane

Use correct syntax for viewport meta tag

Supply viewport metadata for retina displays

Resolves #73

Update eslint to the latest version 

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

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Update webpack-dev-server to the latest version 

Build in production mode for gh-pages

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Otherwise npm ignores devDependencies

This improves React performance by skipping prop type checks and warnings

Update dependencies to enable Greenkeeper 

Also remove outdated notice about PRs

Cache stage bounding box and avoid unnecessary library rendering

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Previously modal content was rendered before the modal was use, causing a re-render on every spriteInfoReport.

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

Make Compatibility Mode on By Default

Make Insertion Marker Visible

Change Toolbox Categories to match new design color

Design Changes

Adding Data to backdrop.json