Do the heavy lifting of document loading on a background queue

[ktraunmueller]

DocumentModel.load() currently calls SourceModel.load() on the main queue; this may cause UI hangs.

Dispatch the work to a background queue and introduce a completion closure.

Edit: maybe also look into https://developer.apple.com/documentation/appkit/nsdocument/1515216-canconcurrentlyreaddocuments

[ktraunmueller]

CRing.tex is an example of a large document that causes a longer hang when opening.