This seems to work fine with current version of wxPopupTransientWindow
and wxPU_CONTAINS_CONTROLS style and allows to drastically simplify the
whole mess of preprocessor checks in wxComboCtrl code, reducing the
logic to just 2 cases: either wxUSE_POPUPWIN == 1 under any of the major
platforms and then we use wxPopupTransientWindow or wxUSE_POPUPWIN == 0
or we use some other platform and then a TLW-based fallback is used
instead.
In spite of the amount of changes, this should only modify behaviour
under MSW where wxPopupTransientWindow is used now instead of whatever
was used before.
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