Set loading state when switching between modes

[koesie10]

This sets the loading state when switching between application and framework mode. It does this by setting the methods to an empty array, which will show the loading state (methods.length === 0).

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
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[robertbrignull]

In practice this works just fine.

The only thing I'm wondering is if it's ok that the methods in ModelEditor and in ModelEditorView are getting temporarily out of sync. Another option could be that when handling the switchMode message, we set ModelEditorView.methods = [] and emit a setMethods message. Ultimately it doesn't matter much because we then call loadExternalApiUsages which updates the methods.

In this case I think this is ok because the methods get recomputed immediately. But it is something to be aware of generally that we need to keep this state in sync.

[koesie10]

Good point, I think setting the methods on the ModelEditorView is better, so I'll update the PR to do that instead.