Dispatching a message through redux makes it reconsider all currently
listening components. Dispatching for each block, while loading, can
mean dispatching and reconsidering thousands of times if not more.
While dispatching is pretty quick doing this for every block ends up
taking a lot of time.

Add a way for GUI consumer to retrieve snapshots of the stage

Update scratch-l10n to the latest version 

Update scratch-vm to the latest version 

Request a toolbox refresh after switching between tabs to fix #4561

Mouse move drag perf

Extracting the drawable art data is very expensive. We should delay
doing that work as long as possible. As such it should wait until a few
more branching return statements before we extract it.

Update scratch-vm to the latest version 

Fix loading single frame gifs by removing onDone in favor of indices

Because the `onFrame` callback could do async work, it was possible for onDone to try to submit the costumes before the onFrame had produced any. This was subject to browsers different timing treatments of setTimeout vs. promise resolution. This makes the gif-decoder use the same style of progress API as the file uploader

Update scratch-l10n to the latest version 

Show an "Importing..." alert during file import

Put the ask blocks text input above the monitors on the stage.

chore(package): update scratch-vm to version 0.2.0-prerelease.2019021…

chore(package): update scratch-render to version 0.1.0-prerelease.201…

Closes #4548

Closes #4544

chore(package): update scratch-l10n to version 3.1.20190213142844

Update scratch-vm to the latest version 

Support isDiscrete sliders

Closes #4536

Prompt before file upload replaces project

Update scratch-vm to the latest version 

chore(package): update scratch-blocks to version 0.1.0-prerelease.154…

Closes #4540

Update scratch-vm to the latest version