Update passive grabs tracking modifiers

In the case of GTK's link widget which doesn't handle passive grabs
following FVWM's window bindings (which assumes the underlying
application can handle a PointerReplay), update the grabs in this
context by always ungrabbing the button and grabbing it for modifiers
other than ALL_MODIFIERS.

I've had this in testing for three years and recently dusted it off for a
bug report I received for another application with this problem, called
darktable.
* Added a comment about GetNextSimpleOption().
Merge remote-tracking branch 'cvs/branch-2_6' into branch-2_6