PowerGUI has an official SDK that allows users to extend Script Editor functionality via Add-ons. You're lucky , Kirk Munro has created an add-on you need.
Simply click Tools -> Find Add-ons Online menu item.. Type Clear in the text box, click Search button, select the add-on and click Install... button. You will get a new menu item inside 'View' top level menu. You may then customize the menu/toolbars (Tool->Customize...) and put the item where you want it to see.
Another problem, I cannot stick it on the toolbar. When I drag it onto the toolbar, it can work well. But when I reopen the script editor, it's missing. Well, it doesn't matter anyway, since I can use Ctrl + R instead.