GSimpleAction: add default activate handler

If the action is stateful and the user doesn't have their own activate handler
then do some reasonable things for ourselves.

After a lot of experience using stateful GSimpleAction it turns out that
people almost always end up using it in the same ways:

A boolean-typed stateful action with no parameter is most likely going
to want to be toggled.  Any other type of action that has the parameter
type equal to the state type probably intends for activation to
represent a request to change the state.

This patch implements those two cases.  This will let people stop
writing their own trivial handlers over and over.

https://bugzilla.gnome.org/show_bug.cgi?id=722503