Commit GUT directly to the git repo and remove gd-plug as it is no
longer required to install GUT.
Modify GUT to be used by command-line only.
For example:
```
cp test/.gutconfig.ci.json .gutconfig.json
godot --no-window -s addons/gut/gut_cmdln.gd
```
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.
- Add TerminalSettings resource.
- Init/load terminal settings from terminal panel.
- Add terminal context menu (i.e. right-click PopupMenu).
- Add shortcut to open a new terminal Ctrl+Shift+T and make terminal panel
visible.
Gut was freezing on some integration tests. It was also entering an
infinite loop after exiting (even after closing Godot and VSCode) which
caused a `godot.log` file in app_userdata to keep growing until my hard
drive was full.
Fresh clones of this repo will need to install Gut using
gd-plug:
```
godot --no-window -s plug.gd install
```
Similar to how git submodules also need to be installed.
