This reverts commit c061f2e2, reversing
changes made to 6e007b1e.

Comments slightly tweaked from kchadha's suggestions

Co-Authored-By: Karishma Chadha <kchadha@media.mit.edu>

This includes a small bug where the target pane wouldn't automatically update if changes to the sprite occured while in fullscreen/player mode.

Closes #4221

Use the same implementation as onSetProjectThumbnailer which does the same thing for getting a thumbnail externally.

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.

Since this is in a promise chain, it doesn't matter if onUpdateProjectData returns a promise or a primitive, it'll work either way