ui::Widget::Base: added get_root_weak(); doc tidy.
txt::Cursor::erase(): corrected sign of size and sane-ified bound check.
ui::ProtoGrid::content_action_internal(): fixed behavior with disjoint_after.¹

1: Inserting after the end of the grid while the view is positioned at the
end of the grid would result in a disjoint_after, which short-circuited
before calling set_row_count().
test/ui/grid: added cell editing; corrected cell focus handling on insertion.