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.

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

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

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.

Since we will be adding more interactivity, the active state will
eventually pop the menu open, so I'm not worried about accessibility of
this right now.

It claims to support other sample rates, as the spec requires, but it
does not.

This is a long standing issue with how iOS deals with scrollable
elements within a fixed area. This was a helpful resource
https://davidwalsh.name/scroll-iframes-ios

Showing/hiding 'more options' in the different types of variable prompts based on var type provided from scratch blocks

This, along with the related VM pull request, fixes LLK/scratch-vm#865.
This adds the GUI code which posts wheel scroll data to the VM's
mouseWheel IO (which, in turn, activates relevant "when key pressed"
blocks).