Add loading indicators to document-creator

[begedin]

Similar to the share-dialog issue #109 , creating a document can take a few seconds. Currently, the create button is removed while the creation process is in progress and a static message is displayed, but this might make it look like the interface hangs, so it would be a good idea to add a loading/progress indicator here as well.

[venkatd]

This sounds like a good idea. Keep it simple if you can.