FlexBoxLayout: fix min-width/min-height axis confusion for column direction

[dfaure-kdab]

flexbox_layout_info() selected cells/padding/spacing using a
(direction, orientation) match. The intention was to pick the
main-axis cells for a main-axis query and cross-axis cells for a
cross-axis query. But the match was wrong:

(Column, Vertical) → main axis → should use cells_v (heights)
but selected cells_h (widths)
(Column, Horizontal) → cross axis → should use cells_h (widths)
but selected cells_v (heights)

For a column layout with items wider than tall (e.g. width=80px,
height=40px), this caused:

• min-height reported as 80px (max item width) instead of 40px
• min-width reported as 40px (max item height) instead of 80px

Fix: select cells/padding/spacing by orientation alone — height
queries always use cells_v, width queries always use cells_h,
regardless of flex direction.

The bug was visible in flexbox-interactive.slint when selecting
Flex Direction: Column, and trying to shrink the window height
as much as possible - it didn't shrink all the way to the height
of the one row of items.