Make research tree vertically scrollable

BuildController.gd

@@ -282,6 +282,8 @@ func _unhandled_input(event: InputEvent) -> void:
 	if skill_menu.visible:
 		if event is InputEventScreenTouch:
 			skill_tree.handle_touch(event)
+		elif event is InputEventScreenDrag:
+			skill_tree.handle_drag(event)
 		# Handled either way. A touch that lands on the panel but misses a
 		# node is still a touch ON THE PANEL - letting it through moved the
 		# fleet and pinch-zoomed the world behind the menu, because
@@ -409,6 +411,7 @@ func _on_action_pressed() -> void:
 
 	if current_building.building_type == GameState.BUILDING_MOTHERSHIP:
 		skill_menu.visible = true
+		skill_tree.reset_scroll()
 		_refresh_skill_menu()
 	elif current_building.building_type == GameState.BUILDING_SHIP_CRAFTER:
 		# Remembered explicitly: _process() clears current_building while any