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.

examples/menu/menu.tscn

@@ -1,7 +1,7 @@
 [gd_scene load_steps=3 format=3 uid="uid://brjrtf5fpptw8"]
 
-[ext_resource type="Script" path="res://addons/godot_xterm/terminal.gd" id="1"]
 [ext_resource type="Script" path="res://examples/menu/menu.gd" id="2"]
+[ext_resource type="Theme" uid="uid://b7vd50tw2g1nl" path="res://themes/default.tres" id="2_o1653"]
 
 [node name="Menu" type="Control"]
 layout_mode = 3
@@ -12,13 +12,11 @@ grow_horizontal = 2
 grow_vertical = 2
 script = ExtResource("2")
 
-[node name="Terminal" type="Control" parent="."]
+[node name="Terminal" type="Terminal" parent="."]
 layout_mode = 1
 anchors_preset = 15
 anchor_right = 1.0
 anchor_bottom = 1.0
 grow_horizontal = 2
 grow_vertical = 2
-focus_mode = 1
-script = ExtResource("1")
-copy_on_selection = false
+theme = ExtResource("2_o1653")