Changed approach for making Panel's "close" button not submit forms. After a code review we decided that it would be better to adjust the "close" button's `template` instead of creating a custom `action` fn. Fixes #2531936

Panel's default "close" button now prevents default on the button event. Panel's "close" button was simply calling the panel's `hide()` method, which allowed the button's default click action to still happen. When a Panel is rendered inside a form this is an undesirable consequence because the `<button>` element will submit the form.

Panel's "close" button config is using the `action` String support.

Renamed Panel.js -> panel.js