Update Devguide marketplace apps and app security content for 6.2

Hi Jesse, if you would please review the changes I made. I walked through adding the Event Listing Portlet project as an app, to update the screenshots. I kept James Falkner's screenshots of his app that is active on marketplace for showing the active app material.

I removed some of the images that I thought weren't needed, in order to make the section better for print layout.

I updated the security sections after testing the policy generation tool. And I refactored the PACL props section to remove what's already on docs.liferay.com.

Thanks for reviewing.