Rather of calling .exists() synchronously for all documents, use
ShellDocSystem's async API to only stat docs we're showing.
Use the doc opening functionality in ShellDocSystem.
Also, more intelligently do redisplay(); don't recreate actors
for recent docs if we already have one for that URI. We may
need more intelligent caching here; if e.g. just the associated
application changes, we should probably reread the info.
https://bugzilla.gnome.org/show_bug.cgi?id=603522
949f67469c