Filter keyboard state against modfifier default mask

Filter keyboard state against modfifier default mask

The modifier state does in fact incorporate modal keys such as
NumLock. So code assuming that state be 0 if no modifier is
pressed is wrong, if NumLock, CapsLock and the like are active.

The solution is to filter out these modal keys with
gtk_accelerator_get_default_mod_mask(), and compare the result.

This fixes the problem of being unable to type keys used as
single-letter shortcuts.