- implements welcome to the preview modal, and unsupported browser modal.
- will need assets updated in Jan.

* Implement GH-830: Add coming soon tooltips

This implements coming soon tooltips for:
* the small stage size
* save, load and language

It also creates the component that can be copied over into the paint editor for use there as well.

Lastly, this does a small refactor on the full screen implementation to include multiple small screen sizes too.

This is for https://github.com/LLK/scratch-paint/pull/194, making the selector area smaller on tablet so as to create more space for the paint editor

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

* Design pass on libraries to make sprites look more like they do in the SpriteSelector. Fixed scrolling overflow when reaching the bottom
* Started a colors variables file
* Added a light transform on sprite item hover

Related #12

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.