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

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.

* 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.

/ht @rschamp

Resolves #73

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

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.

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

Now possible because we send down the correct content type for SVG assets.

Move layout styles from component stylesheets to the GUI, or the component that is placing the component.

Add a new `Box` component to control layouts, basically giving a convenient way to lay out with flexbox.

Use `Box` as the output component for any component that may need to be passed layout options. The props passing convention allows giving layout props to the final `Box` component, making wrapping with another `Box` unnecessary in some cases.

It was not being used by GUI so no reason for it to be initialized there

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.