Update autoprefixer to the latest version 

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Configurable SpriteSelectorItems per row (#90). Standardized spacer variable (#84). MenuBar Pass (#130)

* Standardized use of space units where appropriate
* Reversed box classes for easier readability in Inspector
* Sprite Area: removed unused info button
* SpriteSelectorItems: fixed spacing between rows. Isolated layout + component CSS, now reusable in other context. Items per row is now easily configurable
* MenuBar: refactored structure for clickable items

Update autoprefixer to the latest version 

Update postcss-loader to the latest version 

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Update react-style-proptype to the latest version 

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* Remove unnecessary dev dependencies

* Use large range on true "dev" dependencies

The exact versions of these dependencies are not important for building, so give them large ranges in order to quiet down greenkeeper.

I did not split them between dependencies and devDependencies because a package's dependencies are installed whenever the package is installed as a dependency. Installing all of these would be obnoxious and unnecessary.

Update eslint to the latest version 

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Using postcss-modules-values with imported values failed on our current basic usage; it is apparently not mature enough to be used yet.

The syntax used by postcss-simple-vars is already familiar to most people familiar with modern CSS build systems.

chore(package): update autoprefixer to version 6.7.4

Update xhr to the latest version 

# Conflicts:
#	package.json

# Conflicts:
#	package.json

Update eslint to the latest version 

chore(package): update webpack-dev-server to version 2.4.1

Update svg-url-loader to the latest version 

chore(package): update postcss-loader to version 1.3.1

chore(package): update babel-core to version 6.23.1

chore(package): update babel-loader to version 6.3.2

Update eslint-plugin-react to the latest version 

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Closes #99

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Closes #112

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Closes #82

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Check for empty sound array

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Closes #95

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* 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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Closes #92

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Dynamic menus for costumes and backdrops