Move letters / numbers to top of the sprite library

* Remove errant conditional logic for backdrop / costume modal. Resolves GH-100
* Remove unused var. Re GH-100

* Stage/Backdrop: design pass. Changed how selected state works to keep width of pane from shifting. Switched to fixed pixel width to match design spec
* SpriteSelector: removed flex basis width. Pane grows to expand the empty space, since Stage is now fixed width
* SpriteInfo: updated to correct dropdown values. Updated 2nd row type sizing and radio box border-radius
* SpriteInfo: made labels non-selectable. StageArea: added border
* SpriteSelectorItem: Updating type size to match SpriteInfo

Filter toolbox to blocks available to the VM

Resolves #16

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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