popovermenu: Cycle around focus also with (Shift+)Tab

popovermenu: Cycle around focus also with (Shift+)Tab

Currently when moving the focus with (Shift+)Tab, it also traverses the window's
widgets, although it would be expected that the focus stays within the popover,
as it's (almost) like it's a separate window. This would be consistent with
the behaviour of the Up/down arrows, which do cycle around the focus once it
reaches the end.

So this commit makes the popovermenu cycle around focus in any direction, apart
from left/right because they are used to open and close submenus and it wouldn't
make sense anyway to cycle horizontally as there's usually only one widget per
line.