Monkeypatch dynamic sound menu into ScratchBlocks

/ht @rschamp

Implement shouldComponentUpdate on TargetPane

Use correct syntax for viewport meta tag

Supply viewport metadata for retina displays

Resolves #73

Update eslint to the latest version 

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

https://greenkeeper.io/

Update webpack-dev-server to the latest version 

Build in production mode for gh-pages

https://greenkeeper.io/

Otherwise npm ignores devDependencies

This improves React performance by skipping prop type checks and warnings

Update dependencies to enable Greenkeeper 

Also remove outdated notice about PRs

Cache stage bounding box and avoid unnecessary library rendering

https://greenkeeper.io/

https://greenkeeper.io/

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.

Make Compatibility Mode on By Default

Make Insertion Marker Visible

Change Toolbox Categories to match new design color

Design Changes

Adding Data to backdrop.json

No one else is working on this over here at the moment, so I am going to pull myself. Sorry ya'll.

Add Contribution Guidelines and Issue / PR Templates

Improve media library performance

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