entryArea: Share a single NickPopover

As outlined in the previous commit, there is no real need for
keeping hidden popovers for every room around all the time.
Just share a single popover between all entryAreas to cut down
on the number of widgets (and the resulting overhead of GTK+
updating their state etc.).
1 job for wip/fmuellner/singleton-nick-popover in 3 minutes and 25 seconds (queued for 2 seconds)
Status Name Job ID Coverage
passed build-polari #1292