Further progress towards Godot 4.0 support

- Primary example scenes (menu, terminal, and asciicast) working but
  still a lot of warning/error messages and some regressions.
- Editor integrated terminal works, but still a lot of warning/error
  messages and some regressions.
- Added support for "blink" display attribute.
- Removed GDScript terminal code. Terminal node is now purely a
  GDExtension. So is LibuvUtils.
- GUT tests not working yet.
- Still a lot of things to fix.
- So far, only built for and manually tested on Linux x86_64.

addons/godot_xterm/nodes/terminal/viewport.gd

@@ -1,2 +0,0 @@
-@tool
-extends SubViewport

addons/godot_xterm/nodes/terminal/viewport.tscn

@@ -1,23 +0,0 @@
-[gd_scene load_steps=2 format=3 uid="uid://c62updkby54f3"]
-
-[sub_resource type="GDScript" id="GDScript_d8lvm"]
-script/source = "@tool
-extends SubViewport
-"
-
-[node name="SubViewport" type="SubViewport"]
-transparent_bg = true
-handle_input_locally = false
-gui_snap_controls_to_pixels = false
-size = Vector2i(2, 2)
-render_target_clear_mode = 1
-script = SubResource("GDScript_d8lvm")
-
-[node name="Terminal" type="Terminal" parent="."]
-anchors_preset = 15
-anchor_right = 1.0
-anchor_bottom = 1.0
-offset_right = -2.0
-offset_bottom = -2.0
-grow_horizontal = 2
-grow_vertical = 2