[applets/clipboard] Improve Fitts' law following when deleting multiple items

When you delete multiple items individually, you can keep clicking
without moving your mouse because the delete button doesn't change its
position (except for when the scrollbar hides, but this is inevitable).
However this ceases to be true for tall entries containing files,
images, URLs, or previews, because the buttons are always centered
vertically in the item. So you have to re-position your mouse to click
on the delete button in its new position, and then do so again to
account for the height of the next item, and so forth.

This commit fixed that problem by top-aligning the row of hover buttons,
but only for tall items. For default-height items, they remain
vertically centered.