Duplicate costume

Add duplicate button. Pass the costume ID to paint (the previous costume ID used matches when duplicate is used.) When switching to the costume tab, open the current costume.

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