Columns can now be best-fit resized by double-clicking their
grabbable edges. When a default width is set and all data is empty,
double-clicking will restore the column to its original state.
This is basically just for consistency, it's quite strange to see
multiple AK container types next to each other, some with and some
without the namespace prefix - we're 'using AK::Foo;' a lot and should
leverage that. :^)
If you tried to move a cursor down when the last row is selected, the
index becomes invalid without updating the selection. On the next
cursor movement the invalid index is then reset to {0, 0}, selecting
the first row instead.
