Sets the function used for filtering items.
If match_func is null, the filter matches all items.
If the filter func changes its filtering behavior, gtk_filter_changed needs to be called.
If a previous function was set, its user_destroy will be called now.
| this |
a `GtkCustomFilter` |
| match_func |
function to filter items |
| user_data |
user data to pass to |
| user_destroy |
destroy notify for |