Add focus document toggle

[timon-schelling]

No description provided.

[Keavon]

• Properties panel doesn't populate after toggling Focus Document in the case when nothing is selected.
• Data panel never populates so it's completely broken.
• After closing the last document to return to the welcome screen, the Properties panel doesn't depopulate.
• Alt+Enter doesn't work on the welcome screen, but only sometimes. Seems to happen if you create a document, draw a layer, then close the document. Doesn't happen if you create a new document and either close it immediately or draw something then delete the layer then close it.