Ignore WXK_NONE events in wxStyledTextCtrl.

Ignore WXK_NONE events in wxStyledTextCtrl.

Scintilla use of 0 indicating "modifier key" conflicts with our use of
WXK_NONE indicating absence of a valid key code. As Scintilla can't do
anything with the keys without a key code anyhow, simply ignore them
immediately, without passing them to Scintilla, in DoKeyDown().

This fixes handling of IME input in wxStyledTextCtrl under MSW and possibly
other problems (e.g. with dead char keys).

Closes #13570.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69857 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775