Fix wxKeyEvent::m_uniChar for EVT_CHAR for native controls in wxGTK.

Fix wxKeyEvent::m_uniChar for EVT_CHAR for native controls in wxGTK.

It was mistakenly set to the same value as in EVT_KEY_DOWN event but, just as
m_keyCode, it may be different for EVT_CHAR.

Use gdk_keyval_to_unicode() to set it correctly instead of inheriting
EVT_KEY_DOWN value.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72267 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775