Import input now shows error state w/orange border around input and tooltip with static error message.

Initial SB2 Import UI (button in preview modal + import modal that can load projects via url). Preliminary validation on url, still needs error handling.

- Call scrollToCategoryByName('My Blocks') after custom block is created
- Cannot assume custom blocks section will be last category
- May pose issue during localization efforts

Use the same drag threshold as from the blocks. It may need to be dialed
in further, but testing will be needed.

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

This, along with the related VM pull request, fixes LLK/scratch-vm#865.
This adds the GUI code which posts wheel scroll data to the VM's
mouseWheel IO (which, in turn, activates relevant "when key pressed"
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.