Add a main screen plugin to the plugin demos

plugins/addons/main_screen/README.md

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+# Main Screen Plugin Demo
+
+This plugin demo shows how to make a main screen plugin.
+The main screen appears as a button next to the "2D", "3D", "Script", and
+"AssetLib" buttons. It also shows up when a Node it `handles` is selected.
+
+For more information, see this documentation article:
+https://docs.godotengine.org/en/latest/tutorials/plugins/editor/making_main_screen_plugins.html
+
+If you would like to see a more complete example of what main screen plugins
+are capable of, check out the [2.5D demo project](../../../misc/2.5d).

plugins/addons/main_screen/handled_by_main_screen.gd

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+extends Node

plugins/addons/main_screen/main_panel.tscn

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+[gd_scene load_steps=2 format=2]
+
+[ext_resource path="res://addons/main_screen/print_hello.gd" type="Script" id=1]
+
+[node name="MainPanel" type="CenterContainer"]
+anchor_right = 1.0
+anchor_bottom = 1.0
+size_flags_vertical = 3
+
+[node name="PrintHello" type="Button" parent="."]
+margin_left = 472.0
+margin_top = 290.0
+margin_right = 552.0
+margin_bottom = 310.0
+text = "Print Hello"
+script = ExtResource( 1 )
+__meta__ = {
+"_edit_use_anchors_": false
+}
+[connection signal="pressed" from="PrintHello" to="PrintHello" method="_on_PrintHello_pressed"]

plugins/addons/main_screen/main_screen_plugin.gd

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+tool
+extends EditorPlugin
+
+const MainPanel = preload("res://addons/main_screen/main_panel.tscn")
+
+var main_panel_instance
+
+func _enter_tree():
+	main_panel_instance = MainPanel.instance()
+	# Add the main panel to the editor's main viewport.
+	get_editor_interface().get_editor_viewport().add_child(main_panel_instance)
+	# Hide the main panel. Very much required.
+	make_visible(false)
+
+
+func _exit_tree():
+	if main_panel_instance:
+		main_panel_instance.queue_free()
+
+
+func has_main_screen():
+	return true
+
+
+func make_visible(visible):
+	if main_panel_instance:
+		main_panel_instance.visible = visible
+
+
+# If your plugin doesn't handle any node types, you can remove this method.
+func handles(obj):
+	return obj is preload("res://addons/main_screen/handled_by_main_screen.gd")
+
+
+func get_plugin_name():
+	return "Main Screen Plugin"
+
+
+func get_plugin_icon():
+	# Must return some kind of Texture for the icon.
+	return get_editor_interface().get_base_control().get_icon("Node", "EditorIcons")

plugins/addons/main_screen/plugin.cfg

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+[plugin]
+
+name="Main Screen Plugin Demo"
+description="Demonstrates how to make a main screen plugin."
+author="Aaron Franke, Julian Murgia"
+version="1.0"
+script="main_screen_plugin.gd"

plugins/addons/main_screen/print_hello.gd

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+tool
+extends Button
+
+func _on_PrintHello_pressed():
+	print("Hello from the main screen plugin!")