fgui_event.h: keep keycodes below 256 (fix lineedit bug)
fgui_event.h: keep keycodes below 256 (fix lineedit bug)

Unicode Private Use Area (PUA) seems to be for printable characters, so
move the custom (arrow) key codes into a control code area, where
isprint() does return 0.

This fixes a bug in lineedit where arrow keys would not move the cursor
but insert non-printable characters into the buffer (isprint() fails).

bjornfor/fgui