Obtains a list of keycode/group/level combinations that will generate $keyval. Groups and levels are two kinds of keyboard mode; in general, the level determines whether the top or bottom symbol on a key is used, and the group determines whether the left or right symbol is used. On US keyboards, the shift key changes the keyboard level, and there are no groups. A group switch key might convert a keyboard between Hebrew to English modes, for example. Gtk2::Gdk::Event::Key contains a group field that indicates the active keyboard group. The level is computed from the modifier mask.