custom_tag_start
Description:
[ NoWrapper ]
public abstract bool custom_tag_start (Builder builder, Object? child, string tagname, out BuildableParser parser, out void* data)
  public abstract bool custom_tag_start (Builder builder, Object? child, string tagname, out BuildableParser parser, out void* data)
Called for each unknown element under `<child>`.
Parameters:
| this | a `GtkBuildable` | 
| builder | a `GtkBuilder` used to construct this object | 
| child | child object or null for non-child tags | 
| tagname | name of tag | 
| parser | a `GtkBuildableParser` to fill in | 
| data | return location for user data that will be passed in to parser functions | 
Returns:
| true if an object has a custom implementation, false if it doesn't. |