Fix deleting in sprite selector item event bubbling

This is required because you can delete the last sound, and without a re
render the tab will continue to render the sound editor, causing an
error.

Update gui to use getter for target costumes

Move active tab state into a reducer

Greenkeeper updates 2 22 18

Closes #1450

Closes #1461

Closes #1445

Remove enableExtensions prop

Fixed issue #1409

Update scratch-paint to the latest version 

Fix issue 1212

Detect wheel event on canvas and post data to VM

update my fork

Apply a drag threshold to make clicking sprites easier

Update dependencies to enable Greenkeeper 

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

Fix numerous draggable images in the GUI

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

Update react-modal to the latest version 

Images are draggable by default, but when used within buttons or as
clickable elements, they should not be draggable because starting a drag
accidentally prevents the click event.