Switch to existing document in new tab by default

Right now Okular defaults to opening an already-open document in a new
tab when opened a second time, which is a bit odd. Most other
document-based apps will switch to the existing document's instance
instead of opening a new instance of it.

Interestingly enough, Okular has an option to do this, but it's off by
default. I think on by default makes more sense here, to be consistent
with other document-based apps. Since the option exists, people who have
a specialized use case that commonly involves opening multiple instances
of the same document can toggle the option to have that. Everyone wins!

BUG: 432942
FIXED-IN: 21.04