Editor palettes: allow keyboard scrolling to see the last row

[stevecotton]

Fixes #6724, where scrolling with the mousewheel could reach the last row, but
scrolling with keys or buttons couldn't. Although can_scroll_up() and
can_scroll_down() already existed, can_scroll_down() didn't allow access to the
last line when that line had less than the full number of items. The mousewheel
still worked, because it ignored that logic.

Remove a feature that was seen when the last row is visible - things jumped
between columns to fill the spaces. If you have 10 items, with 4 columns and 2
visible rows then it will appear like this:

    a b c d
    e f g h

when scrolling down it used to move items sideways to fill the last line, which
seems bad UI because it made items harder to find:

    c d e f
    g h i j

the new code is simpler, and will instead show the following when scrolling down:

    e f g h
    i j

Move the layout code into adjust_size(), so that it runs one time when the number
of buttons changes. That could be separated from this commit, but the code
would still be touched in this commit (counter_from_zero would still be
replaced by i), so doing it and testing the changes together made sense.

Note for the master branch only: scrolling the palettes up and down is
noticeably laggy, which is a regression from last week. That's not caused by
this commit, we're just at a point in the rendering refactor where
surface_restorer::update() and thus gui::widget::hide() are slow. The fix
for that is already in progress and going to be in 1.17.5.

[Wedge009]

I confirm the slow-down in the editor scrolling was already present in master. I also confirm this change accomplishes what I would expect the behaviour of the tiles to be when scrolling - thanks for that.