_gdk_x11_keymap_key_is_modifier() never tries to set min/max_keycode
if they haven't been set before, meaning that until another function
sets those, all the keys will be seen as non-modifiers.

This causes GdkKeyEvents to be wrongly tagged with "->is_modifier = 0"
when in actual fact the key is a modifier. This fixes keyboard
shortcuts captured with GtkCellRendererAccel in "raw" mode thinking
a modifier without any actual keys is a valid shortcut.

https://bugzilla.gnome.org/show_bug.cgi?id=670400