Push to history when the library mounts. Whenever popstate occurs after mounting, request to be closed.

Behavior captured in the `Modal` container. If we want to re-use this functionality for anything else that uses `ModalComponent`, just use the `Modal` container rather than `ModalComponent`.

Update the vm to include fixes for downloading imported sounds

Merge pull request #1832 from LLK/revert-1829-greenkeeper/scratch-paint-0.2.0-prerelease.20180420154430

Revert bitmap paint editor PR for now

Update chromedriver to the latest version 

chore(package): update scratch-paint to version 0.2.0-prerelease.2018…

Update scratch-vm version.

Library tweaks

This won't work on a lot of browsers, but temporarily just target modern ones. (Robust solution is possible using classNames and refs)

Closes #1760

Add USE_HEADLESS=no npm run test:integration option

Libraries Phase II

Thanks @towerofnix!

Including action-menu which was also giving one when the backdrop library opened.
Localize the costume library, I missed it while doing the other libraries.

Update scratch-blocks to the latest version 

They're spammy — happy to put this back but they were unhelpful while fixing tests IMO.

chore(package): update scratch-blocks to version 0.1.0-prerelease.152…

Reorder and add default arguments to gt, lt, eq blocks

Adjustments to string operator default arguments

Thanks @ericrosenbaum!