Handles the display of the inbox or outbox

• Loads up all the information for the specific folder, and checks to see if you're only displaying messages with a specific label
• Sets up the page index
• Checks to see if you can send PMs
• Uses the 'folder' sub-template if you aren't using a wireless protocol
• Calls messageIndexBar() to load the sidebar
• Calls prepareMessageContext() with appropriate parameters and loads the results into $context['get_pmessage']
• Loads up all the recipient info into the $recipients global array
• Accessed at index.php?action=pm or index.php?action=outbox