Area2D and Area use same functions, but with different names

[qarmin]

Godot version:
3.2 master

Issue description:
Function to set/get audio_bus_name property use
in Area function set_audio_bus
and in Area2D function set_audio_bus_name

[akien-mga]

The Area methods should be renamed to include _name, so that they match the property. Moving this to #16863 as it's compat breaking.