Makes for pretty paths when extending scripts:
`extends "res://addons/godot_xterm/terminal.gd"`
vs.
`extends "res://addons/godot_xterm/nodes/terminal/terminal.gd"`
Currently "res://addons/godot_xterm/pty.gd" is acutally `pty_unix.gd`.
This is okay for now as the PTY node is only supported on Unix
platforms. However, we will need to sort it out when adding Windows
support as part of #25.
Also remove the GDXterm namespace.
- Don't add the bell to the archive to keep it small a simplify
licensing. Also bells seem to be rarely used with terminal emulators.
- Don't play the bell directly from the Terminal node by adding an
AudioStreamPlayer, but make it easy to tune the "bell" signal behavior
from the Terminal node so that only an AudioStreamPlayer node's play()
method needs to be connected to it.
- Keep the bell.wav sound around for testing/demo.
Currently tests only run on X11.64 platform.
But other platforms can be supported with a bit of effort.
Remove default bell sound as it does not play nicely with CI environment
that does not have sound card.
