Added first setup of level-based system

UI/Menus/MainMenu/level_selecter_main_menu.gd

@@ -0,0 +1,30 @@
+class_name MainMenuLevelSelector extends PanelContainer
+
+signal level_selected(specs: LevelSpecs)
+
+var side: int = 0
+
+@export var levels: Array[LevelSpecs] = []
+
+func _on_button_pressed() -> void:
+	side = 0
+	$VBoxContainer/Label.text = "Standard Campaign Mode"
+
+
+func _on_button_2_pressed() -> void:
+	side = 1
+	$VBoxContainer/Label.text = "Endless Mode with random waves and all unlocked equipment"
+
+
+func _ready() -> void:
+	var i: int = 0
+	for level: LevelSpecs in levels:
+		i += 1
+		var button: Button = Button.new()
+		button.text = "Level " + str(i)
+		$VBoxContainer.add_child(button)
+		button.pressed.connect(start_level.bind(i - 1))
+
+
+func start_level(level: int) -> void:
+	level_selected.emit(levels[level])