Add a control gallery demo

This project showcases what Godot's various Control nodes can do
(and what they look like, so that new users can figure them out).

gui/control_gallery/project.godot

@@ -0,0 +1,34 @@
+; Engine configuration file.
+; It's best edited using the editor UI and not directly,
+; since the parameters that go here are not all obvious.
+;
+; Format:
+;   [section] ; section goes between []
+;   param=value ; assign values to parameters
+
+config_version=4
+
+_global_script_classes=[  ]
+_global_script_class_icons={
+
+}
+
+[application]
+
+config/name="Control Gallery"
+config/description="Showcases various Control nodes with their names affixed for easy recognition."
+run/main_scene="res://control_gallery.tscn"
+run/low_processor_mode=true
+config/icon="res://icon.png"
+
+[display]
+
+window/dpi/allow_hidpi=true
+window/stretch/mode="2d"
+window/stretch/aspect="expand"
+
+[rendering]
+
+quality/driver/driver_name="GLES2"
+vram_compression/import_etc=true
+vram_compression/import_etc2=false