array: Add a clear function

array: Add a clear function

Like GPtrArray has a "free function" that can be used to free memory
associated to each pointer in the array, GArray would benefit from
having a "clear function" that can be used to clear the content of
each element of the array when it's removed, or when the entire array
is freed.

https://bugzilla.gnome.org/show_bug.cgi?id=667243