Due to lack of support for horizontal scrolling in the native control,
scrolling it was implementing by offsetting the entire control window.
However this didn't work correctly when the window was not positioned
at the leftmost border of its parent window, as the part of it that was
scrolled off could still be visible in this case, and this is exactly
what happened when the native header was used in wxGrid: scrolling it
overwrote the corner part of wxGrid.
Fix this by embedding the actual native control inside an outer wxWindow,
to ensure that the scrolled off part is clipped by the parent window.
Note that this commit is best viewed with "git show --color-moved" as
most of the code was just moved from the header into the implementation
file and is not really new.
120b33746a