Handle Alt+Esc or Esc+Esc in InputAlt correctly 

If another `ESC` byte appears when termbox is in `InputAlt` and an `ESC` byte was already detected, the byte is just discarded. This leads to Alt+Esc being ignored.

This commit fixes this behaviour: If Alt+Esc is pressed in InputAlt mode, an event with `{Key: KeyEsc, Mod: ModAlt}` is generated. This also happens if Esc is pressed twice.