UI: Support left-right arrow key walk navigation in UI lists
Add improved arrow key walk navigation in grid layout UI List templates.
Pressing up or down walks the active item to the adjacent row in that
direction, while left and right walk through the items along the columns
wrapping at the rows.
Note from Julian:
In combination with the following commit, this has the important
limitation that the list's custom activate operator won't be called when
"walking over" an item that is scrolled out of the list. That is because
we don't actually create any buttons for those that could be used for
the handling logic. For our purposes of the pose libraries that should
be fine since the asset view list is always made big enough to display
all items. Solving this might be difficult, we don't properly support
nesting boxes with proper scrolling in regular layouts. It's all just
hacked a bit for UI-lists to work. Overlaps quite a bit with T86149
Differential Revision: https://developer.blender.org/D11063