scrolledwindow: Only render overshoot on the widget window

Without any check, it was being rendered also on the indicator
GdkWindows, which resulted on an over-exposed effect there.
gtkpopover: Fix allocation of child widget wrt margins

During size request, all sides' margins are ensured to be as big
as TAIL_HEIGHT, just to avoid possible relocation loops if the
popover doesn't fit in its original position. This must be
accounted for in size_allocate() as well.
scrolledwindow: Don't doubly draw overlay-indicator scrollbars

The scrollbars are still internal children of the GtkScrolledWindow
and handled in ::forall(), no need to propagate draw here again
after chaining up in ::draw().
Updated Norwegian bokmål translation.
scrolledwindow: Use a fully transparent background for the overlay window

The background will be already given and updated in the GtkScrollbar widget,
plus the background remained static on the window since it is realized.