Refactor code that setups mapping tables between imported xdg foreign toplevels and transient surfaces

zxdg_importer_v2_import_toplevel has a couple of connect blocks, one with a nested connect, that are slightly hard to read. This change introduces two functions - link() and unlink() that reduce the size of the connect blocks and improve readability.

link() is used to setup a mapping between the specified parent and the child.

unlink() is used to break a mapping with the specified parent or a child.