Add GUT (CLI only) and remove gd-plug

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
```

addons/gut/icon.png.import

@@ -0,0 +1,35 @@
+[remap]
+
+importer="texture"
+type="StreamTexture"
+path="res://.import/icon.png-91b084043b8aaf2f1c906e7b9fa92969.stex"
+metadata={
+"vram_texture": false
+}
+
+[deps]
+
+source_file="res://addons/gut/icon.png"
+dest_files=[ "res://.import/icon.png-91b084043b8aaf2f1c906e7b9fa92969.stex" ]
+
+[params]
+
+compress/mode=0
+compress/lossy_quality=0.7
+compress/hdr_mode=0
+compress/bptc_ldr=0
+compress/normal_map=0
+flags/repeat=0
+flags/filter=true
+flags/mipmaps=false
+flags/anisotropic=false
+flags/srgb=2
+process/fix_alpha_border=true
+process/premult_alpha=false
+process/HDR_as_SRGB=false
+process/invert_color=false
+process/normal_map_invert_y=false
+stream=false
+size_limit=0
+detect_3d=true
+svg/scale=1.0