Don't set Unicode key code in key events to non-Unicode values in wxGTK.

Don't set Unicode key code in key events to non-Unicode values in wxGTK.

Only assign the wx key code wxKeyEvent::m_uniChar if it's a key corresponding
to an ASCII symbol, don't do it for the values outside of ASCII range such as
all the special WXK_ constants. It doesn't make sense to generate Unicode key
codes for e.g. cursor key presses.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65240 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775