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.
- Enable tests for osx.universal platform.
- Export HTML5 (but skip upload as .cast files aren't exported
properly).
- HACK: Use HTML5 export to generate .import directory for tests
This way we don't get an error message even if using
`continue-on-error` set to `true`.
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.
