Showing/hiding 'more options' in the different types of variable prompts based on var type provided from scratch blocks

Remove the WebGL checks from the error boundary. Instead, just do not
render the stage if it is not supported.

* Added object.assign polyfill

* Fix IE load failure

Don’t call `(window.AudioContext || window.webkitAudioContext)()` if platform is IE

Catch any other errors, and display Browser unsupported in error-boundary.

If for some reason there are no other errors (unlikely), preview-modal will still check for IE and show the unsupported modal.

Polyfilling Object.assign is enough to get through load and show the modal for IE. We can always switch to `babel-polyfill` if there are others needed later, but for now this is smaller.

temporary patch for #1205.

Add Error Boundary (has componentDidCatch method) to handle WebGL errors and display an informative message. 

* Implement paint-75: Add blank costume button

* add backdrop icon, use default name

thanks @fsih !

* Add coming soon extensions to the library

This implements #1074

* don’t call `onSelect` when disabled

thanks @paulkaplan!

* add speech image to coming soon extension

* add feedback form

This implements #829

* small style updates to the stage buttons

from @carljbowman

* remove `stageSize` for now

We eventually need this, but not yet, and it’s causing lint failure.

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

Fixes #993

Fixes https://github.com/LLK/scratch-gui/issues/1064

* 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.

Localizes the pen extension:
- loads all pen locale information - may want to consider dynamically loading locales later.