entryArea: Share a single NickPopover
The popover for changing the nick is currently handled entirely as part of the entryArea, which means we have to keep one popover for every room around, even though they don't carry any room-specific state and we never show more than one at any given 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.).