GAction: make GObject properties read-only

GAction: make GObject properties read-only

GAction is a read-only interface (as is visible by the lack of _set() functions
on its API).  The properties on the interface currently force implementors to
support writing of the properties at construct time, however.

Lift that restriction.

Take advantage of this from GSimpleAction by nuking the set_property
function and setting the fields directly in the constructor.