Add gdk_window_get_children_with_user_data

Add gdk_window_get_children_with_user_data

This function returns all the children that has a specific user_data set.
This is used a lot in the new GtkWidget drawing code and doing
it this way is faster than getting every child and calling get_user_data
on each (which was a non-neglible part of the profiles). Additionally it
also allows use to use some kind of hashtable to make this operation even
faster if needed in the future.