Sharing costumes and sounds to other sprites and the stage

[paulkaplan]

Resolves

What Github issue does this resolve (please include link)?

Resolves #1983
Resolves #2395

Proposed Changes

Describe what this Pull Request does

Allows target pane to handle "drop" events from the costume/sound tiles, which duplicates the costume/sound to that sprite.

Of note:

• You can share assets to the sprite that already owns that asset ("yourself"). This duplicates the asset. This is an existing behavior of scratch 2...
• This does not work on touch yet because of the reliance of hoveredTarget on mouseEnter/mouseLeave events, same issue impacting sharing blocks. I'm working on a fix.
• In order to prevent reorders from happening when sharing assets, I confined the reorder logic in sortableHOC to only work when you are over the sortable areas container. This might make reordering feel weird, but I can't think of another way to prevent them both from happening. Let's keep an eye on this in feedback/testing /cc @carljbowman.

[kchadha]

Looks good!