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Fixes #156 + refactored classnames to style guide conventions

* Buffer sprite info text fields

Changes to the name, x, or y fields should only be sent to the VM when the field is blurred or when enter is pressed.

Still allow updates while the fields are focused, but unchanged.

Fixes #154

* Remove focus from input fields after hitting enter

* Fix prop types for info component and container

SpriteInfo: Design + refactor pass
* Updated SVG icons
* Added tab indexes, related #88. Still no tab on radio buttons
* Removed outline on dropdown focus
* Radio icons: fixed redirecting issue, using `div` instead of `a`. Temporary, until radio buttons. Related #156
* All icons: unselectable to prevent highlighting. Radio icons clickable
* SpriteSelector: fixed bottom padding in scroll wrapper

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

* 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

* 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.