Fix text input and completion in wxComboCtrl and wxOwnerDrawnComboBox.

Fix text input and completion in wxComboCtrl and wxOwnerDrawnComboBox.

Both wxEVT_KEY_DOWN and wxEVT_CHAR must be handled and only the latter
used for completion (or any characters input). Don't make incorrect
assumptions about wxEVT_CHAR keycodes either, it's only the Unicode
character and printability that matter.

Otherwise, completion in readonly controls wouldn't work correctly
for e.g. numbers on numpad or non-ASCII characters.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63864 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775