Initial version of translated tutorial gifs

Step Ids and step file names did not follow a consistent pattern. To make it easier to automate in the future, things were renamed in the following ways:

StepId: camel case, prefix with the short name that we usually use to refer to the tutorial, e.g. name, music, chaseGame. Followed by a name for the step, e.g. nameAddSprite, nameAddBackdrop. When there are two of a step, add ‘2’ to the end e.g. nameAddSprite2

file-naming: all step files should be in the steps directory. The name is the ‘kebab’ case of the stepId, followed by the language-code, then the file extension. For example the english version of the file for `nameAddSprite` would be `name-add-sprite.en.gif`, `chaseGameAddSprite2` would be `chase-game-add-sprite2.en.gif`

chore(package): update scratch-vm to version 0.2.0-prerelease.2019022…

Update scratch-l10n to the latest version 

* thumbnails folder: thumbnail images for tutorials (multistep decks and videos). Images should not need translations (avoid text that would need translation in the image)
* steps folder: gif images associated with tutorial steps. Images can be translated. English (default) images have the extension `.en.gif`, other language images should replace the `.en` with the corresponding locale.
* renamed some of the steps files to be more consistent and a bit less redundant
* removed a couple of files that were no longer referenced
* revised webpack config to allow localized step files to be loaded from `/static` on www

Fixes #4253 by telling the vm the langauge when it is initialized.  B…

Closes #4588

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