Replace proto_tree_add_text with proto_tree_add_subtree(_format) in generated files

As requested by Michael Mann, all occurences of proto_tree_add_text/proto_item_add_subtree are replaced with proto_tree_add_subtree(_format) in the generated files. Our generator templates have also been updated accordingly.

Change-Id: I49ddd664dffef4b3ceda77edd1b2d7e01da363f3
Reviewed-on: https://code.wireshark.org/review/4725
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>