Adds unit tests for this HOC.

Use PureComponent instead since it is a simple shouldComponentUpdate check now.

We were emitting drops when getMouseOverIndex was null, which could happen either when the drop is outside the container, or there were zero boxes to check for ordering. Treat these two cases separately, and continue to not emit drops when outside the container.

This reverts commit 97f8eb63, reversing
changes made to 2e017f06.

This is used because the sortable drop is also used for general dropping as well as sorting drop, so you need to be able to drop into a sortable list that does not contain any items.

Make the corresponding reorder code be more careful to check if there is a newIndex.

https://github.com/LLK/scratch-gui/pull/4312 had a function name conflict with the existing code. This change fixes that by renaming the conflicting function, and removes redundant code.

Browsers other than IE will be allowed to load the GUI.
However, if there is an error, the error boundary will display an unsupported browser message recommending they update to the latest version of a supported browser.

For the purposes of this PR, ‘supported’ means you can load the gui (anything other than IE), and recommended is just the browsers that meet our minimum requirements for Chrome, Firefox, Edge and Safari.

The playground now displays this on startup when in Scratch Desktop
simulation mode.

This is because when switching from project to editor view, this component gets unmounted and then remounted, but the componentDidMount seems to happen before the componentWillUnmount. Because it is dealing with a global variable, it is unsafe to always unset the beforeunload when unmounting.

This prevents unnecassary changes that do not matter to the player view because they were made by the project in progress (i.e. made by running the blocks, or by cloud var updates)

They were not being marked as clean when loaded from the server, causing remixes that included only minimal (or no) changes to take a very long time as it waited for hundreds of network requests to resolve. Mark them as clean when loaded from the server so we know they do not need to be resaved

This was intended as a safety check but it causes problems with some
tests.